monolithic zirconia a review of the literature

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Current classification of zirconia in dentistry: an updated review

Suchada kongkiatkamon.

1 Department of Prosthetic Dentistry, Faculty of Dentistry, Prince of Songkla University, Songkhla, Thailand

Dinesh Rokaya

2 Faculty of Dentistry, Zarqa University, Zarqa, Jordan

Santiphab Kengtanyakich

3 Prosthodontic Section, Department of Restorative Dentistry, Naresuan University, Phitsanulok, Thailand

Chaimongkon Peampring

Associated data.

The following information was supplied regarding data availability:

This is a literature review.

Zirconia, a crystalline oxide of zirconium, holds good mechanical, optical, and biological properties. The metal-free restorations, mostly consisting of all-ceramic/zirconia restorations, are becoming popular restorative materials in restorative and prosthetic dentistry choices for aesthetic and biological reasons. Dental zirconia has increased over the past years producing wide varieties of zirconia for prosthetic restorations in dentistry. At present, literature is lacking on the recent zirconia biomaterials in dentistry. Currently, no article has the latest information on the various zirconia biomaterials in dentistry. Hence, the aim of this article is to present an overview of recent dental zirconia biomaterials and tends to classify the recent zirconia biomaterials in dentistry. This article is useful for dentists, dental technicians, prosthodontists, academicians, and researchers in the field of dental zirconia.

Introduction

Zirconia (ZrO 2 ) is a crystalline oxide of zirconium and it holds good mechanical, optical, and biological properties ( Bapat et al., 2022 ). This biomaterial has three basic chemical forms; monoclinic, tetragonal, and cubic ( Saridag, Tak & Alniacik, 2013 ; Bocanegra-Bernal & dela Torre, 2002 ). The metal-free restorations, mostly consisting of all-ceramic/zirconia restorations, are becoming popular restorative materials in restorative dentistry choices for aesthetic and biological reasons ( Kongkiatkamon et al., 2021 ). Recently, there have been significant improvements in restorative biomaterials including dental zirconia, and producing wide varieties of zirconia for prosthetic restorations in dentistry ( Kontonasaki et al., 2019 ; Kontonasaki, Giasimakopoulos & Rigos, 2020 ; Humagain & Rokaya, 2019 ; Amornvit et al., 2021 ). With the advancement of digital technologies, intraoral scanners, and CAD/CAM systems, it has become possible to fabricate dental restorations digitally with easy processing, designing, and high accuracy ( Al-Qahtani et al., 2021 ; Ahmed et al., 2021 ).

Pure zirconia exists in the monoclinic form at room temperature and with an increase in temperature (1,170 °C) or low-temperature degradation (LTD), it transforms to the tetragonal form ( Ban, 2021 ). Further increasing temperature (2,370 °C), aging or hydrothermal aging, progressive transformation to monoclinic phase takes place ( Piconi & Maccauro, 1999 ; Sorrentino et al., 2019 ; Rekow et al., 2011 ) ( Fig. 1 ). Then cooling, the tetragonal form transforms back to the monoclinic form. Achieving stable sintered zirconia ceramic is a little difficult because volumetric change (about 5%) occurs when the transformation from tetragonal to monoclinic. The zirconia can be monochromatic with uniform composition, polychromatic multilayer with uniform composition, and polychromatic multilayer and hybrid composition.

(A) monoclinic; (B) tetragonal; (C) cubic structure; and (D) phases of transformation of zirconia. Modified with permission from Sorrentino et al. (2019) .

Proper bonding between the zirconia restoration and the tooth is important for the longevity of the prosthetic restoration ( Araújo et al., 2018 ; Melo et al., 2015 ; Heboyan et al., 2023 ). Zirconia requires surface treatments with acid etching for surface abrasion to ensure adhesion with luting cement ( Araújo et al., 2018 ). Although there are various surface treatment protocols have been recommended, common treatment included alumina particles followed by the application of primers or cements based on MDP (10-methacryloyloxydecyl dihydrogen phosphate) ( Melo et al., 2015 ; Aung et al., 2019 ; Shimizu et al., 2018 ; Silveira et al., 2022 ; Alammar & Blatz, 2022 ). The surface modification improves the adhesive behavior of the materials ( Silveira et al., 2022 ).

Dental zirconia has increased over the past years producing wide varieties of zirconia for prosthetic restorations in dentistry. Although some researchers have studied zirconia and classified dental zirconia in the past, ( Ban, 2021 ; Güth et al., 2019 ; Nistor et al., 2019 ; Grech & Antunes, 2019 ; Alqutaibi et al., 2022 ) the current literature is lacking on the recent zirconia biomaterials in dentistry. The research question is there a recent classification of the recent zirconia biomaterials in dentistry? It is found that no article has the latest information on the various types of zirconia biomaterials in dentistry. Hence, the aim of this article is to present an overview of recent dental zirconia biomaterials and tends to classify the recent zirconia biomaterials in dentistry. This article is useful for dentists, dental technicians, prosthodontists, and researchers in the field of dental zirconia by providing updated information on the current literatures on various types of zirconia used in dentistry.

Survey Methodology

Articles on advances in dental zirconia ceramic were searched from January 1989 to December 2022 using Google Scholar, MEDLINE/PubMed, Web of Science, and ScienceDirect resources. Research and review articles in the English language were only included in this review. A total of 79 articles were selected and included in this review. Editorials, Letters to the Editor, and Case Reports were excluded from this review.

Yttria stabilized zirconia

Often in zirconia, various elements are dissolved such as yttrium (Y), cerium (Ce), calcium (Ca), magnesium (Mg), etc. to make it stable at room temperature ( Piconi & Maccauro, 1999 ; Chevalier, 2006 ). The addition of Yttria (Y 2 O 3 ) to zirconia stabilizes the tetragonal phase ( Leib et al., 2015 ). Following LTD, yttria is exhausted through reaction causing the phase transformation ( Rekow et al., 2011 ; Chevalier, Cales & Drouin, 1999 ; Amat et al., 2019 ). Yttria-doping can reduce grain growth, stabilize the tetragonal phase, and substantially improve thermal stability. Furthermore, the thermal stability of the cubic form of zirconia is obtained by the substitution of some Zr4+ ions (ionic radius of 0.82 Å) with larger ions, e.g. , Y3+ (ionic radius of 0.96 Å) in the crystal lattice. This doping of zirconia results in partially stabilized zirconia (PSZ) ( Leib et al., 2015 ).

The yttria-stabilized dental zirconia is classified into 12 types ( Fig. 2 ). Zirconia (TZP, tetragonal zirconia polycrystal) are of various types based on the yttria content: ( Zhang, 2014 ; Abdulmajeed et al., 2020 ; Arcila et al., 2021 ) 3Y-TZP (3 mole % Y-TZP), 4Y-TZP (4 mole % Y-TZP), 5Y-TZP (5 mole % Y-TZP), and 6Y-TZP (6 mole % Y-TZP). The 3Y-TZP is early zirconia used in dentistry as a “white metal” ( Miyazaki et al., 2013 ). Zirconia with lower yttria content (3Y-TZP, 3 mole % Y-TZP) has better mechanical properties and less translucency whereas 3Y-TZP (3 mole % Y-TZP) with increased yttria content (6Y-TZP, 6 mole % Y-TZP) has more translucency but presents lower mechanical properties. Yttria content consisting of >8 mol% has a stable cubic phase at room temperature and it is known as cubic stabilized zirconia (CSZ). Similarly, yttria content consisting of 3-8 mol% has tetragonal and cubic phases and it is known as partially stabilized zirconia (PSZ). And yttria content consisting of approx. 3 mol% has tetragonal phases (toughened) about 100% and it is known as a tetragonal zirconia polycrystal (TZP).

Y, Yttria, M, multilayer.

At present multilayer (M) zirconia has been introduced. Similarly, M3Y is highly translucent and M6Y is super highly translucent ( Fig. 2 ). Some surface defects can be seen in all types of zirconia under scanning electron microscopy, although the 3Y-TZP demonstrates higher grain consistency. It has been found that the 5Y-PSZ presents the least strength and the 4Y-PSZ and 3Y-TZP present similar fatigue. It has been found that higher yttria content has lower mechanical strength but higher translucency of zirconia ( Ban, 2021 ; Harada et al., 2020 ; Cho et al., 2020 ). Similar to yttria, ceria (CeO 2 ) is added to the zirconia to produce ceria in tetragonal stabilized zirconia (Ce-TZP).

Properties of zirconia

Physical properties.

Zirconia is a stable restorative biomaterial. Dental zirconia is resistant to acid erosive attacks in the mouth although some erosive agents may have a negative effect on the surface roughness ( Tanweer et al., 2022 ). It has extremely low thermal conductivity and the thermal expansion coefficient is 10 × 10 − 6/°C and does not depend on the yttria content ( Ban, 2021 ).

Mechanical properties

Zirconia has the highest hardness among the various restorative materials used in dentistry ( Ban, 2021 ). Its flexural strength and hardness are extremely large compared to other restorative materials. Conventional zirconia has higher bi-axial flexural strength compared to high-translucent monolithic zirconia ( Kontonasaki, Giasimakopoulos & Rigos, 2020 ). Furthermore, the fracture toughness of 5Y-TZP is almost 50% less compared to that of 3Y-TZP with the cubic phase content because of more yttria content ( Belli et al., 2021 ). In a recent study, ( Liao et al., 2023 ) showed that the flexural strength value was 584 (158) MPa for 3Y-TZP and 373 (104) MPa for 5Y-TZP.

Dal Piva et al. (2018) studied the influence of the milling system and aging on zirconia surface roughness and phase transformation and they found that the surface roughness of zirconia-based crowns was not influenced by the milling system or low-temperature degradation. But regarding the phase transformation, autoclaving and pH-cycling aging presented a monoclinic phase increase when compared to the control group and thermocycled group. Similarly, a study by Flinn et al. (2012) on the accelerated aging of Y-TZP found that the hydrothermal aging of Y-TZP can cause a significant transformation from tetragonal to monoclinic crystal structure with a significant decrease in the flexural strength of thin bars. Hence, the aging of zirconia increases the monoclinic phase.

The fracture strength of a zirconia implant is influenced by its design, composition, and kind of abutment preparation ( Bethke et al., 2020 ). The 1-piece zirconia implant fixture has twice the fracture strength compared to the 2-piece fixture ( Kohal, Finke & Klaus, 2009 ). There is a strong correlation between the fracture toughness and fracture loads of ceramic crowns on zirconia implants during the occlusal contact ( Rohr, Märtin & Fischer, 2018 ). Therefore, proper selection of zirconia material should be done for the crown whether aesthetics or strength is needed.

Zirconia is supposed to cause the opposing teeth to wear. But smooth and well-polished zirconia does not cause tooth wear. Abrasive wear on the occlusal part of zirconia restoration affects the opposing teeth or restoration ( Mair, 1992 ). When the zirconia restoration is a hard and rough surface, the tooth abrasive wear becomes severe.

Optical properties

Zirconia is an esthetic biomaterial, but its translucency is slightly less compared to the glass-ceramics. To maintain the translucency of the zirconia and glass-ceramic prostheses, suitable luting cement should be used ( Heboyan et al., 2023 ; Bilgrami et al., 2022b ; Bilgrami et al., 2022a ). The addition of yttria content in zirconia increases the cubic phases and this increases the translucency, however, the strength is reduced due to a few tetragonal phases ( Fig. 3 ). 5Y-TZP is more translucent by 20 to 25% but has less flexural strength by 40 to 50% compared to 3Y ( Ban, 2021 ). Hence, 3Y-TZP can be indicated for bridges, especially of long spans, and is not suitable for the anterior teeth ( Ban, 2021 ; Liao et al., 2023 ). Conversely, 5Y-TZP and M5Y are indicated for veneers and anterior crowns but are not suitable for long-span bridges ( Ban, 2021 ). Similarly, for the hybrid multilayer and polychromatic zirconia types, such as M3Y-5Y, their uses are similar to 5Y-TZP which has low strength ( Jitwirachot et al., 2022 ). Similarly, both 4Y-TZP and M4Y can be used in all areas requiring sufficient strength and translucency.

Zirconia has greater radio-opacity compared to aluminum and titanium. This is due to its intrinsically high density and effective atoms which can obtain high-contrast radiographic images useful for diagnosis ( Ban, 2021 ). Speed sintering can reduce the translucency of the zirconia. It was found that regular sintering had larger gain sizes and increased translucency than speed sintering ( Kongkiatkamon & Peampring, 2022 ).

Biological properties

Various animal and human studies conclude that zirconia is a biocompatible biomaterial ( Bapat et al., 2022 ; Josset et al., 1999 ; Christel et al., 1989 ; Uo et al., 2003 ; Abd El-Ghany & Sherief, 2016 ; Zarone et al., 2021 ). Christel et al. (1989) investigated the effect of yttria-stabilized zirconia and alumina in vivo (implanted into paraspinal muscles of rats) and found no cytotoxicity. Similarly, Josset et al. (1999) also found that the human osteoblasts presented good adhesion and cell spreading, and the cells maintained their proliferation capacity and differentiation ability into osteogenic pathways. Wu et al. (2015) studied the wettability of ZrO 2 and found that its wettability was substantially enhanced by oxygen plasma treatment for maintaining a stable hydrophilicity surface. Water droplets can wet the hydrophilic zirconia surface (low contact angle) and this wetting condition that is suitable for oil-water separation is achieved by engineering the surface chemistry and surface roughness characteristics ( Rasouli et al., 2021 ). Hydrophilic surface is an important factor that affects protein absorption and human gingival fibroblasts’ cellular attachment to implant abutments ( Rutkunas et al., 2022 ; Barberi & Spriano, 2021 ; Kim et al., 2015 ) Generally, a lower contact angle promotes fibroblast attachment ( Kim et al., 2015 ).

Furthermore, zirconia does not cause mutations in the cellular genome ( Silva, Lameiras & Lobato, 2002 ; Warashina et al., 2003 ). Moreover, ZrO 2 creates a less toxic reaction in tissue compared to titanium ( Degidi et al., 2006 ). Zirconia also shows less bacterial adhesion and it is important in maintaining good periodontal health ( Scarano et al., 2004 ). It was found that zirconia showed less adhesion of bacteria and less biofilm formation compared to titanium ( Ban, 2021 ; Scarano et al., 2004 ; Rimondini et al., 2002 ). Scarano et al. (2004) found that bacterial adhesion was 12.1% on zirconia vs to 19.3% on titanium.

Sintering of zirconia

CAD/CAM technology used computer-aided design and fabrication of ceramic prostheses and the process is more time efficient than conventional techniques ( Padrós et al., 2020 ; Abduo & Lyons, 2013 ). Sintering is responsible for providing the strengths to the zirconia restoration. Various sintering methods have been developed and they affect the structure, properties, and esthetics of zirconia ( Kilinc & Sanal, 2021 ; Juntavee & Attashu, 2018 ; Sanal & Kilinc, 2020 ). Different studies compared different (slow and fast) sintering protocols of zirconia ( Amat et al., 2019 ; Kilinc & Sanal, 2021 ; Ordoñez Balladares et al., 2022 ; Ersoy et al., 2015 ; Liu et al., 2022b ). Juntavee & Attashu (2018) studied the role of sintering duration and temperature on the mechanical properties of zirconia and found that a long sintering time with high sintering temperature results in increased flexural strength zirconia. Similarly, Kongkiatkamon & Peampring (2022) evaluated the surface microstructure, flexural strength, and translucency of 5Y-TZP zirconia using regular and speed sintering. They found that the regular protocol showed bigger gain sizes and more translucency than the speed protocol. The speed sintering had higher biaxial flexural strengths which can be due to changes in the material structure from the degradation of the metal salts ( Sulaiman et al., 2017 ). Similarly, Liu et al. (2022a) also found that the Y-PSZ with conventional sintering had a bigger average grain size and fewer fine grains compared to the speed sintering of zirconia. Ahmed et al. (2020) found no dimensional change between normal and fast sintering of zirconia. Liu et al. (2022b) investigated the optical properties of 3Y-TZP and 5Y-TZP and noticed that speed sintering had less lightness without affecting the surface roughness.

Surface treatment and adhesion of zirconia

Bonding between resin cement and zirconia is difficult to achieve because of their chemical inertness and lack of silica content ( Scaminaci Russo et al., 2019 ). Hence, surface treatments of the zirconia restoration increase the adhesive, micro tensile bond strength, and longevity of the prosthetic restoration ( Araújo et al., 2018 ; Melo et al., 2015 ; Heboyan et al., 2023 ). At present various surface treatments for zirconia and ceramics are available for better bonding to the tooth structure ( Campos et al., 2016 ; Guarda et al., 2013 ; Sato et al., 2016 ). Airborne-particle abrasion and tribo-chemical silica coating are the pre-treatment methods. Adhesion can be increased after physicochemical conditioning of zirconia ( Scaminaci Russo et al., 2019 ). One common treatment includes alumina particles followed by the application of primers or cement-based on10 MDP (methacryloyloxydecyl dihydrogen phosphate) ( Melo et al., 2015 ; Aung et al., 2019 ; Shimizu et al., 2018 ; Silveira et al., 2022 ; Alammar & Blatz, 2022 ) However, the effect of the bond strength with the new generation of high-translucent zirconia materials is not clear and further studies are needed.

Classification of zirconia

The previous classifications of zirconia were done according to the types of polycrystalline (zirconia, Alumina, PSZ, TZP, and yttria-stabilized dental zirconia; Generation 1–3) ( Sato et al., 2016 ). Zirconia can be of various types as shown in Table 1 . Commonly, zirconia can be uniform or hybrid in composition and monolayer or multilayer.

Since there are various ceramic materials in the market and it is often confusion regarding choosing the material. Hence, the authors would like to categorize the zirconia materials based on their composition, and an indication of the commercially available zirconia materials ( Table 2 and Figs. 4 – 5 ).

A, 3Y-TZP, B, 4Y-TZP, and C, 5Y-TZP.

Finally, this review article provides updated information on the various dental types of zirconia used in dentistry. As this review does not use the PICO method, this review can be extended to do a more extensive review following the PICOS method.

Conclusions

Zirconia can be of various types based on the yttria content, uniform or hybrid composition, monochromatic or polychromatic, and monolayer or multilayer. Increased yttria content in zirconia results in higher translucency but reduces the strength. Zirconia with lower yttria content (3Y-TZP, 3 mole % Y-TZP) has better mechanical properties and less translucency whereas 3Y-TZP (3 mole % Y-TZP) with increased yttria content (6Y-TZP, 6 mole % Y-TZP) has more translucency but presents lower mechanical properties. Speed sintering of zirconia has resulted in higher flexural strength and regular sintering of zirconia has shown bigger gain sizes and more translucency.

Funding Statement

The authors received no funding for this work.

Additional Information and Declarations

Dinesh Rokaya is an Academic Editor for PeerJ.

Suchada Kongkiatkamon conceived and designed the experiments, performed the experiments, analyzed the data, prepared figures and/or tables, authored or reviewed drafts of the article, and approved the final draft.

Dinesh Rokaya conceived and designed the experiments, prepared figures and/or tables, authored or reviewed drafts of the article, and approved the final draft.

Santiphab Kengtanyakich analyzed the data, authored or reviewed drafts of the article, and approved the final draft.

Chaimongkon Peampring analyzed the data, authored or reviewed drafts of the article, review, and approved the final draft.

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Monolithic Zirconia: An Update to Current Knowledge. Optical Properties, Wear, and Clinical Performance

Affiliations.

• 1 Department of Prosthodontics, School of Dentistry, Faculty of Health Sciences, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece. [email protected].
• 2 Private, Cambridge CB1 3NU, UK.
• 3 Private, Limassol 3061, Cyprus.
• 4 Private, 55131 Thessaloniki, Greece.
• PMID: 31480688
• PMCID: PMC6784470
• DOI: 10.3390/dj7030090

The purpose of this paper was to update the knowledge concerning the wear, translucency, as well as clinical performance of monolithic zirconia ceramics, aiming at highlighting their advantages and weaknesses through data presented in recent literature. New ultra-translucent and multicolor monolithic zirconia ceramics present considerably improved aesthetics and translucency, which, according to the literature reviewed, is similar to those of the more translucent lithium disilicate ceramics. A profound advantage is their high strength at thin geometries preserving their mechanical integrity. Based on the reviewed articles, monolithic zirconia ceramics cause minimal wear of antagonists, especially if appropriately polished, although no evidence still exists regarding the ultra-translucent compositions. Concerning the survival of monolithic zirconia restorations, the present review demonstrates the findings of the existing short-term studies, which reveal promising results after evaluating their performance for up to 5 or 7 years. Although a significant increase in translucency has been achieved, new translucent monolithic zirconia ceramics have to be further evaluated both in vitro and in vivo for their long-term potential to preserve their outstanding properties. Due to limited studies evaluating the wear properties of ultra-translucent material, no sound conclusions can be made, whereas well-designed clinical studies are urgently needed to enlighten issues of prognosis and long-term survival.

Keywords: clinical performance; monolithic zirconia; translucency; wear.

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Conflict of interest statement

The authors declare no conflict of interest.

Correlation between absorption coefficient (…

Correlation between absorption coefficient ( α ) and thickness ( z ) and…

Correlation of translucency and strength…

Correlation of translucency and strength of contemporary zirconia and lithium disilicate ceramics.

SEM atomic-number contrast backscattered electron…

SEM atomic-number contrast backscattered electron image of a cross-sectioned glazed monolithic zirconia specimen…

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Monolithic zirconia: A review of the literature

2016, Biomedical Research-tokyo

Veneer cracking or chipping is the major complication of the zirconia based restorations. Monolithic zirconia has been introduced to overcome this problem, as well as to use in patients with limited interocclusal space. Many research articles on monolithic zirconia crowns have been published in the last years. The aim of this review article was to present data about the wear, surface roughness, fracture strength, optical properties, and marginal fit of monolithic zirconia. A PubMed search was conducted with the terms of “zirconia” with “monolithic”, “full-contour”, “solid”, “translucent”, “anatomiccontoured”, “un-veneered”, “non-veneered”, “full-coverage”. Based on the results of these studies, monolithic zirconia crowns with polished surfaces have been shown to cause the lowest wear on the antagonists compared to glazed zirconia. The fracture strength of monolithic zirconia has been found higher than veneered zirconia. Monolithic zirconia may be a promising future and long-term fol...

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Biotribology

João Caramês

The main aim of this study was to perform a scoping review on the wear behaviour of recent dental glass-ceramics and its influence on the damage of opposing tooth enamel surfaces. Relevant in vivo and in vitro studies reported significant damage on enamel surfaces after friction against current glass-ceramics and therefore different parameters have been studied such as the glass-ceramic type, wear set up, environment, and surface treatment. The opposing enamel loss in volume, weight, or vertical measurement shown by previous studies is significantly higher when compared to enamel-by-enamel wear values. In fact, high values of hardness and elastic modulus of glass-ceramics combined with rough surfaces can result in detrimental effects on the tooth enamel surfaces. Restorative dental materials must withstand masticatory loads in combination with aesthetic benefits and high biocompatibility. Nevertheless, elastic modulus, hardness, and roughness of glass-ceramics should be adjusted to decrease the damage on tooth enamel surfaces.

Journal of prosthodontics : official journal of the American College of Prosthodontists

GUILLERMO JESUS PRADIES RAMIRO

To compare the surface roughness and biaxial flexural strength of dental ceramics obtained after chairside surface modification by mechanical polishing procedures, versus laboratory reglazing. Discs (16 × 1.5 ± 1.6 mm) (N = 90) of various framework-veneering combinations were fabricated: D/FC: lithium disilicate/feldspathic ceramic; Z/AL: zirconium dioxide/aluminous ceramic; N/FC: noble alloy/feldspathic ceramic; N/FF: noble alloy feldspathic with fluorapatite; B/FC: base alloy/feldspathic ceramic; B/FF: base alloy/feldspathic ceramic with fluorapatite. In each group 10 specimens were ground using a diamond bur (46 μm) and five were polished with silicone-reinforced disc polishers (25 μm). Surface roughness (Ra) was measured using contact profilometry. After thermocycling in artificial saliva (6000 cycles, 5 to 55 ± 5°C), biaxial flexural strength was measured using "piston-on-three ball" test. The data (N) were analyzed using one-way ANOVA, Bonferroni, and Tukey's pos...

Se-wook Pyo

Computer-aided design and manufacturing technology has been closely associated with implant-supported restoration. The digital system employed for prosthodontic restorations comprises data acquisition, processing, and manufacturing using subtractive or additive methods. As digital implantology has developed, optical scanning, computer-based digital algorithms, fabricating techniques, and numerical control skills have all rapidly improved in terms of their accuracy, which has resulted in the development of new ceramic materials with advanced esthetics and durability for clinical application. This study reviews the application of digital technology in implant-supported dental restoration and explores two globally utilized ceramic restorative materials: Yttria-stabilized tetragonal zirconia polycrystalline and lithium disilicate glass ceramics.

The International Journal of Prosthodontics

Hyeonjong Lee

Brazilian Dental Science

Hanaa Zaghloul

Dental Materials

Florian Beuer

Journal of Applied Oral Science

Thaís Montanheiro

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Custom-made versus prefabricated zirconia crowns for primary molars: A 12-month follow-up

Alassar, Roqia Mohammad 1,2 ; Metwally, Noha Ibrahim 3 ; Abdelgawad, Asmaa Mohammad 1 ; Elsherbeny, Selwan Hassan 3 ; Mohamed, Eman Abdelraouf 3,4

1 Department of Crowns and Bridges, Faculty of Dental Medicine for Girls, Al-Azhar University, Cairo, Egypt

2 Department of Fixed Prosthodontics, Faculty of Dentistry, Pharos University in Alexandria, Alexandria, Egypt

3 Department of Pedodontics and Oral Dental Health, Faculty of Dental Medicine for Girls, Al-Azhar University, Cairo, Egypt

4 Department of Pedodontics, Faculty of Dentistry, Sinai University, Kantara Branch, Ismailia, Egypt

Address for correspondence: Dr. Noha Ibrahim Metwally, Youssef Abbas Street, Nasr City, Cairo 11651, Egypt. E-mail: [email protected]

This is an open-access article distributed under the terms of the Creative Commons Attribution-Noncommercial-Share Alike 4.0 Unported, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Background: 

Many practitioners have questioned whether the construction method of pediatric zirconia crowns impacts the periodontal health and clinical performance of severely decayed primary molars. The objective of this study was to compare the periodontal health and clinical performance of primary molars restored with custom-made zirconia crowns (CZCs) and prefabricated zirconia crowns.

Methods: 

Twenty primary molars indicated for crown restorations were selected from ten patients (5–9 years old) randomly. Each patient received two pediatric zirconia crowns constructed by two different methods: one custom-made and one prefabricated. The primary molars were divided into two groups: Group 1: primary molars received CZCs and Group 2: primary molars received prefabricated zirconia crowns (PZCs).

Results: 

After a 12-month follow-up, there was no statistically significant difference between the periodontal health of primary molars restored with custom-made and prefabricated zirconia crowns. The clinical performance of primary molars restored with CZCs was statistically significantly higher than those restored with PZCs in terms of retention and fracture resistance ( P ≤ 0.05).

Conclusions: 

The construction method of pediatric zirconia crowns does not significantly affect the periodontal health of primary molars; however, clinical performance is significantly affected in terms of retention and fracture resistance.

Clinical Significance: 

A CZC is an excellent alternative option, especially for primary molars whose permanent successors still have a long time to erupt. The PZC is a quick and easy restoration, but the technique is sensitive.

Introduction

Nowadays, restoring badly decayed primary molars with crowns that guarantee better periodontal health and maximum esthetics has become a clinical demand. Although the longevity of stainless steel crowns (SSCs) is higher than conventional restorations, unfortunately, SSCs are frequently refused by parents for esthetic reasons. [ 1-3 ]

Zirconia is rapidly becoming the most preferred dental material for adults’ and children’s crown restorations due to biocompatibility, durability, and esthetics. Prefabricated zirconia crowns (PZCs) have been introduced since 2008 [ 4 , 5 ] and compared to SSCs in previous studies. [ 6-8 ] However, there is a gap in the literature about the effectiveness of custom-made zirconia crowns (CZCs) in primary teeth. [ 9 ] Therefore, CZCs were suggested to obtain better marginal adaptation along with maximum crown fitness.

The present study aims to compare the periodontal health and the clinical performance of primary molars restored with CZCs and PZCs. The null hypothesis was divided into two parts; the first part was that the periodontal health of primary molars restored with CZCs would be equivalent to those restored with PZCs, and the second part was that the construction method of pediatric zirconia crowns had no influence on the clinical performance of primary molars.

This clinical trial, which was randomized and followed up for 12 months, received approval from the Research Ethics Committee at the Faculty of Dental Medicine for Girls, Al-Azhar University. The protocol ID for the study was PD-P-020-006.

Sample size calculation and patient selection

The G*Power statistical power analysis program (version 3.1.9.7) was utilized to determine the sample size. It was found that a total sample size of 20 (with 10 in each group) was adequate to detect an effect size of ( F = 1.371) with an actual power (1−β error) of 0.8 (80%) and a significance level (α error) of 0.05 (5%). [ 6 ]

A total of 10 patients, with the age range of 5–9 years old, were selected according to the established inclusion and exclusion criteria, in conjunction with the departments of pedodontics and crowns and bridges. The patients’ guardians were informed about the purpose of the investigation, the clinical procedures, and the advantages of the applied materials and techniques. Written informed consent forms were signed by patient guardians, and verbal assent from the children was also obtained before study initiation. Inclusion and exclusion criteria for sample selection were set as the following.

Inclusion criteria

• Participants are children aged between 5 and 9 years
• Each child has a minimum of 2 primary molars with decay, which are indicated for crown restorations
• The presence of dental caries extending to the middle third of the dentin is confirmed both clinically and radiographically
• There is a willingness to participate and adhere to the follow-up appointments
• Teeth in the opposite arch with normal occlusion are present.

Exclusion criteria

• Children who have systemic health issues
• Children suffering from periodontal disease or exhibiting parafunctional habits
• Children who have a unilateral chewing habit or an allergy to local anesthesia.

Patient education

The participating children continued with their routine dental appointments in the undergraduate training clinic. Before beginning, children and their parents received a standard oral hygiene education from an undergraduate student through a brushing demonstration on a model. [ 10 ] Parents were warned when signs of gingival inflammation are present, or plaque control is inadequate.

Samples grouping

Twenty decayed primary molars indicated for crown restorations were selected from 10 wpatients (5–9 years old) included in this study. Each patient received at least two pediatric zirconia crowns constructed by two different methods: one custom-made and one prefabricated as illustrated in Table 1 . A randomized controlled trial was designed, in which the primary molars were randomly assigned to a specific crown group ( n = 10) according to the method of crown construction using the split-mouth design so that each subject forms its own control, adjusting for potential confounders: [ 11 ]

• Group 1: Primary molars received CZCs
• Group 2: Primary molars received prefabricated zirconia crowns (PZCs).

Clinical procedures

Custom-made zirconia crowns group.

Tooth preparation

Tooth preparation was performed by tapered diamond with a round end (850-314-016, Komet, Germany) under local anesthesia, with the following guidelines: [ 6 , 12 , 13 ] 1.5 mm occlusal reduction, 1 mm axial reduction, and 0.5 mm chamfer finish line established 1 mm subgingivally [ Figure 1 ].

Taking the impression

An impregnated double zero retraction cord (Ultrapak E, Ultradent, USA) was used to ensure proper soft-tissue management and an accurate impression. In plastic stock trays, the impression was taken with vinyl polysiloxane addition silicon (Elite Hd+A Silicone, Zhermack, Italy). A double-mix two-step impression technique was used: the putty viscosity was used with a spacer, the spacer was removed, and the light viscosity was applied using an impression gun to ensure a homogenous mix was achieved. After setting, the impression was removed from the patient’s mouth with a snap removal and checked.

Provisional phase

SSC was used as a temporary crown until the customized zirconia crown was fabricated. The SSC was contoured and trimmed with scissors to extend up to 1 mm subgingivally before being cemented temporarily using zinc oxide eugenol cement (Dentotemp, Itena, France).

Steps of custom-made zirconia crown construction

Master cast fabrication

The impression was poured with scannable type dental stone (type IV, GC FUJIROCK EP, GC, America) according to the manufacturer’s instructions, mixing and pouring were done with a vacuum.

The master cast was scanned using an extraoral scanner (DOF, Seoul, Republic of Korea). The casts were attached to the scanner’s rotary arm. The scan sequence was as follows: upper arch, lower arch, and finally, the two casts in occlusion.

The virtual model became accessible to the designing software Exocad (Exocad software, Germany Dental CAD 2.4). The restoration parameters were adjusted after selecting monolithic zirconia as the restorative material, determining the margin, and selecting an appropriate path of insertion. An internal gap to provide space for cement was kept constant at 50 μ. The designed crowns were saved as an STL file that was sent to the milling machine.

A 5-axis milling unit machine (Imes-IcoreCORiTEC, GmbH Germany) was used for milling of Katana™ Zirconia ML blank (Kuraray Noritake Dental Inc. Japan).

Sintering process

The crown was then placed in a special furnace (Sirona HTC, DenSply, Canada), with zirconia sintering beads (Z beads, Ltd., China) in a crucible crack tray (Alien crucible tray, Glendora, California), with the occlusal surface facing down to complete the sintering process. The furnace was adjusted to 1500°C/2732°F with a hold time of 2 h and a rate of temperature increase of 10°C/18°F min, rate of temperature −10°C/−18°F − 10°C/−18°F min. This stage was required to achieve the restorations’ final flexural strength and dimensions.

Characterization and glazing

The intaglio surface of the crown was sandblasted by alumina air particles (Korox 50, Bego, Bremen, Germany) (50 μm, 30 psi, 0.2 Mpa) using a sandblasting machine (Cobra, Germany). An ultrasonic cleaner in alcohol was used to clean the restorations. The surface of the zirconia crown was smoothed as fine as possible using a silicone diamond point (polierer–set Zirko-Shine, oko dent Gmbh and Co. KG, Germany) then a glazing material (CERABIEN™ ZR FC Paste Stain, Kurary Noritake Inc., Japan) was applied.

Prefabricated zirconia crowns group

Preparation was performed according to the manufacturer’s instructions, [ 6 , 11 , 12 ] by reducing the occlusal surface uniformly by 0.6-1.0 mm, axial surface by 0.5-1.0 mm, and rounding of point-and line angles with a circumferential featheredge finish line established 1 mm subgingivally [ Figure 2 ].

Selection of a prefabricated zirconia crown (PZC)

The preparation was adjusted to passively receive a suitable size of prefabricated zirconia crown (Elephant Dental Supplies, BIBODENT, Taiwan). The PZC is selected according to the MD dimension of the primary molar tooth. The selected size allows the crown to fit the preparation properly. After gingival bleeding subsided, the selected PZC was cemented in the same treatment session.

Checking and verification

All the custom-made and prefabricated crowns were checked for complete seating, marginal fit, and occlusion in centric and eccentric positions.

Cementation

All crowns were cemented using glass-ionomer cement (Medicem, Promedica, Germany). First, a zirconia surface cleaner (ZirClean; BISCO, USA) was applied to the fitting surface of the crown and left for 20 s to improve the surface energy at the intaglio. The cement was of proper consistency when it was pulled into a thread of about 20 mm in length before snapping back to the slab. The excess was removed with a sharp probe. Interproximal waxed dental floss was used to remove residual cement between the crown and the adjacent teeth. Furthermore, after the initial setting of the glass ionomer cement, an articulating paper was utilized to check for any occlusal interference that required corrections. Patients were advised to perform routine tooth brushing using the proper technique.

Evaluation methods and follow-up strategy

Periodontal health was assessed using plaque index (PI) [ 6 , 14-16 ] and gingival index (GI). [ 6 , 7 , 13 , 14 ] Only the sample teeth were evaluated giving a PI and GI score for each tooth surface. The means of four surfaces (mesial, distal, buccal, and lingual) for each tooth were used as the PI and GI scores [ 6 ] as illustrated in Table 2 . The PI and GI scores were measured at the different time intervals designed for the study, so that each patient was evaluated in the same appointment for both crowns. Baseline measurements were scored 1 week after custom-made and prefabricated crowns were cemented. Basically, CZC in each patient was prepared and constructed first, allowing the cementation of both types of crowns in the same visit.

Clinical performance was assessed in terms of retention, marginal adaptation, crown contours, and fracture resistance according to the Modified United States Public Health Service (MUSPHS) Criteria, [ 17-19 ] as illustrated in Table 2 .

The patients were recalled for evaluation at a baseline and at 1-, 3-, 6-, and 12-month follow-ups. The evaluation was performed by two different examiners: a pedodontist and a fixed prosthodontist.

Study outcomes

The primary outcome was the change in periodontal health at baseline (t0) and postoperatively at 1 (t1), 3 (t2), 6 (t3), and 12 (t4) months. The primary outcome was the success of the treatment. The absence of major failures such as crown dislodgement, open margin, or crown fracture defined success. [ 15 , 20 ] The secondary outcome was the change in clinical performance postoperatively at 1 (t1), 3 (t2), 6 (t3), and 12 (t4) months. The clinical success of the crowns was assessed in terms of crown retention, marginal adaptation, crown contour, and fracture resistance.

Statistical analysis

The statistical analysis was conducted using IBM® SPSS® (Version 25.0; IBM Corp, 1 New Orchard Road, Armonk, New York 10504-1722, United States), and Microsoft Excel 365. The Shapiro–Wilk test for normality was employed to examine all quantitative data for normality. The data, which were found to be nonparametric, were presented as median and interquartile range values in Tables 2 - 7 . The Mann–Whitney test was utilized to compare different parameters between the groups with custom-made and prefabricated zirconia crowns. The levels of significance were set at P ≤ 0.05.

The results of this clinical study are given in Tables 3 - 8 .

Primary zirconia crowns are now available in two options: either custom-made or prefabricated sizes and shapes. Zirconia crown costs for both options are the same, but custom-made crowns can be more expensive than prefabricated crowns because the time taken to prepare a tooth for a custom zirconia crown is higher and therefore requires temporary restorations using SSC crowns.

CZC has many benefits. First, it presents the exact shape and size of the tooth using computer-aided design/computer-aided manufacturing (CAD/CAM) technology to determine the size of the restorations required to design and mill the specific custom-sized tooth which means that the restoration is of a perfect fit for the patient. Second, due to its adaptive translucent nature, CZCs can be made to suit the specific tooth color of the patient. Third, the chair side work for crown placement, adjustments, and contouring is much less for CZCs. Fourth, according to the current study, this new procedure may lead to an increase in retention and fracture resistance as CZCs have a longer life span in the patients’ mouths than prefabricated ones. [ 9 ] Fifth, the shape and size of CZCs could be adjusted easily mainly in areas of crowding or space loss. [ 12 ]

Prefabricated zirconia crowns are easily available in the market, and it is a quick method when it comes to pediatric dental restorations. They are of various sizes and shapes that may fit most children’s teeth. They have significantly less time to choose a crown and fit it for the patient due to its availability, therefore removing the need for a temporary restoration. [ 21 ] However, they have a slightly higher chair time when compared to custom-made crowns because they have standard anatomy which makes it challenging to fit into different teeth alignments and occlusions. Furthermore, they have a slightly less natural color and come in limited shades. [ 8 , 22 ]

Treatment of carious primary molars with prefabricated zirconia crowns is considered the preferred option by most dentists because zirconia is the most stable, natural, and biocompatible material in the market. [ 6 , 21 , 23 , 24 ] Zirconia crowns are completely metal-free and suitable for kids who have metal allergies specifically to nickel. In two systematic reviews, [ 12 , 25 ] prefabricated zirconia crowns seem to be a viable alternative to preformed metal crowns, offering more advantages in esthetics, retention, fracture resistance, parental satisfaction, and gingival health. Furthermore, a randomized control trial conducted by Taran and Kaya [ 6 ] compared the periodontal health of primary molars restored with prefabricated zirconia and SSCs. They concluded that zirconia crowns outperformed SSCs and controls in terms of gingival health and plaque accumulation.

There is a gap in the literature regarding the restoration of primary molars with custom-made crowns. Although there is a significant body of literature available for adults, little is known about esthetic dentistry for children. [ 26 , 27 ] Multiple sessions and prolonged procedures needed to fabricate custom-made crowns may be the main causes why they are not commonly used.

The results of this study revealed that the periodontal health of primary molars restored with CZCs was not statistically significantly different than teeth restored with prefabricated zirconia crowns (PZCs) at 3-, 6-, and 12-time interval examinations [ Tables 3 and 4 ], while the clinical performance of the CZCs was statistically significantly higher than the PZCs in terms of retention and fracture resistance [ Tables 5 and 8 ]. Therefore, the first part of the study hypothesis was accepted, whereas the second part was rejected.

Although the baseline measurements were scored 1 week after crowns were cemented, the GI of the primary molars restored with prefabricated crowns showed a significantly higher mean rank at baseline examination [ Table 4 ]. This could be explained by the deterioration of the gingival margins of primary molars during tooth preparation, crown selection, and cementation at one visit, which was the main cause of gingival inflammation at baseline examinations. In primary teeth, subgingival margin placement is not preferable; however, retention of full-coverage crowns generally requires subgingival adaptation. In addition, due to the smooth surface of zirconia, a low affinity for plaque accumulation. [ 19 ] According to the study of Yamane et al ., [ 28 ] plaque accumulation by zirconia was not different from that for the enamel-substituted hydroxyapatite.

In this study, the finish line was placed 1 mm subgingival. While it is not ideal to place the margin subgingivally in primary teeth, the retention of full-coverage crowns typically necessitates subgingival adaptation. [ 28 ] The long-term health of the periodontium surrounding crowned teeth is linked to well-fitted seating, marginal contacts, and the absence of cement remnants in the sulcus, all of which contribute to plaque accumulation. [ 29 ] The periodontal health of teeth restored with prefabricated crowns can also decline over time. [ 6 ] Literature reports suggest that while prefabricated crown restorations offer superior aesthetics, they are often associated with poor gingival health, gingival bleeding, and exposure to restorative margins. [ 30 ]

No clinical studies compared CZCs to prefabricated zirconia crowns, but each was compared to prefabricated metal crowns in different studies. [ 6 , 11 , 13 , 31-34 ] Four studies compared the periodontal health around prefabricated zirconia crowns with that around prefabricated metal crowns. [ 31-34 ] None of the studies observed significant gingival inflammation around the prefabricated zirconia crowns. GI scores were significantly lower around prefabricated zirconia crowns compared with prefabricated metal crowns.

In terms of retention, this study found that the prefabricated zirconia crowns had significantly higher crown dislodgment scores compared to the custom-made group, as shown in Table 5 . The majority of the failures were attributed to inadequate preparations and issues with cementation. An in vitro study by Jing et al . indicated that at least 2 mm of tooth structure is required for the retention of prefabricated zirconia crowns. [ 35 ] Furthermore, when installing the prefabricated zirconia crowns, the tooth must be trimmed to ensure that the crown fits passively, which results in lower retention compared to custom-made crowns. [ 4 ]

Regarding fracture, the prefabricated zirconia crowns showed statistically significantly higher fracture scores than the custom-made group [ Table 8 ]. This result could be attributed to two main reasons. First, as reported by the manufacturer, the flexural strength of Katana Zirconia is 1125 MPa to make the material of the highest durability. Second, the different amounts of tooth reduction in both groups, consequently the different crown thicknesses.

In this study, two prefabricated crowns were fractured, one before cementation due to sudden biting and one at baseline due to undercut. These failures did not affect the final sample size because at the time of sample size determination, the authors have added a 25% of the total determined sample size to compensate for the dropouts caused by any reason at different time intervals throughout the study. Elephant prefabricated zirconia crown (Elephant Dental Supplies, BIBODENT, Taiwan) preserves the remaining tooth structure as it needs 0.6 mm for occlusal reduction and 0.4 mm for axial reduction with a featheredge subgingival finish line. The crown is autoclavable so no need for a try-in kit. However, it is a technique sensitive, easily fractured during try-in if the child bites before cementation. Furthermore, fracture occurs if the preparation has any undercut.

This finding is in line with an in vitro study conducted by Elian El Hayek et al ., [ 9 ] which demonstrated that CZCs had a significantly higher fracture resistance than their prefabricated counterparts. The study concluded that the force needed to fracture the primary posterior crowns was significantly higher for the new CZCs compared to the prefabricated ones. These findings may prompt pedodontists to adopt this innovative prosthetic approach in pediatric dentistry. This result is also consistent with a study by Oğuz et al ., [ 36 ] which compared CAD/CAM and prefabricated zirconia crowns for primary molars and concluded that CAD/CAM zirconia crowns, which exhibited the highest fracture resistance and no microcrack formation, may have a longer lifespan.

Contrarily, an in vitro study by El Shahawy and Azab [ 37 ] found no significant difference in the fracture resistance of prefabricated and CZCs. It was further concluded that prefabricated zirconia crowns for permanent molars could perform as well as custom-made crowns for adults in terms of fracture resistance, making them suitable for children and capable of withstanding the occlusal forces of an adult. This discrepancy could be due to the different materials used and the thickness of the finish line.

Conclusions

Within the limitations of this study including limited sample size, comparing only one type of PZCs to CZCs, time constraints, and absence of previous research studies on the same topic, it could be concluded that:

• The periodontal health of primary molars restored with PZCs is comparable to that restored with CZCs
• The construction method of primary zirconia crowns has a significant effect on their clinical performance in terms of retention and fracture resistance
• The clinical performance of primary molars restored with CZCs is better than that restored with PZCs.

To overcome these limitations in future studies, it would be suggested to design wider sample subjects over more prolonged time follow-ups and compare more types of PZCs to CZCs.

Financial support and sponsorship

Conflicts of interest.

There are no conflicts of interest.

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Biomedical Research

Review Article - Biomedical Research (2016) Volume 27, Issue 4

Monolithic zirconia: A review of the literature

Zeynep Özkurt-Kayahan *

Department of Prosthodontics, Faculty of Dentistry, Yeditepe University, Istanbul, Turkey

Accepted date: May 04, 2016

Veneer cracking or chipping is the major complication of the zirconia based restorations. Monolithic zirconia has been introduced to overcome this problem, as well as to use in patients with limited interocclusal space. Many research articles on monolithic zirconia crowns have been published in the last years. The aim of this review article was to present data about the wear, surface roughness, fracture strength, optical properties, and marginal fit of monolithic zirconia. A PubMed search was conducted with the terms of “zirconia” with “monolithic”, “full-contour”, “solid”, “translucent”, “anatomiccontoured”, “un-veneered”, “non-veneered”, “full-coverage”. Based on the results of these studies, monolithic zirconia crowns with polished surfaces have been shown to cause the lowest wear on the antagonists compared to glazed zirconia. The fracture strength of monolithic zirconia has been found higher than veneered zirconia. Monolithic zirconia may be a promising future and long-term follow-up studies are needed to determine whether it may be an alternative to conventional veneered zirconia.

Monolithic zirconia, Full-contour zirconia, Clinical studies, In vitro studies, Review.

Introduction

Zirconia or zirconium dioxide (ZrO 2 ) is a highly attractive ceramic material in prosthodontics due to its excellent mechanical properties related to transformation toughening, which are the highest ever reported for any dental ceramic and enhanced natural appearance compared to metal-ceramics [ 1 - 3 ]. It is widely used to build prosthetic devices because of its good chemical properties, dimensional stability, high mechanical strength, toughness, and a Young’s modulus (210 GPa) similar to that of stainless steel alloy (193 GPa) [ 2 , 3 ].

The high initial strength and fracture toughness of zirconia results from a physical property of partially stabilized zirconia known as “transformation toughening” [ 2 , 3 ]. Zirconia is a polymorphic material that has 3 crystal phases: monoclinic (m), tetragonal (t), and cubic (c). At room temperature, zirconia is in monoclinic phase and transforms into tetragonal phase at 1170°C, followed by a cubic structure at 2370°C [ 2 ]. While cooling, the metastable tetragonal zirconia is transformed into stable monoclinic zirconia. The tetragonal to monoclinic (t→m) phase transformation is associated with a large volume expansion (3-5%) that induces compressive stresses opposing crack opening and acts to increase resistance to crack propagation [ 3 ]. In vitro studies of zirconia specimens demonstrate a flexural strength of 900 to 1200 MPa and a fracture toughness of 9 to 10 MPa/m 2 [ 4 ]. It is a bioinert, not soluble metal oxide [ 5 ] that also exhibits a favorable radioopacity and a low corrosion potential [ 1 ].

Zirconia frameworks can be produced according to two different CAD/CAM techniques. In soft machining technique, CAD/CAM systems shape pre-sintered blocks, which involves machining enlarged frameworks in a so-called green state. The enlarged pre-sintered zirconia frameworks are then sintered in a sintering furnace to their full strength that is accompanied by shrinkage of the milled framework by 25% to the desired dimensions [ 1 ]. In hard machining technique, fully sintered blocks are shaped [ 1 ]. The framework coloration is performed either adding metal oxides to the zirconia powder, or embedding the frameworks in metal salt solutions after machining [ 6 ]. Glazing is created by firing a small coating of transparent glass onto the surface or by heating the framework up to glazing temperatures for 1 to 2 minutes to get shiny glass surfaces [ 7 ].

Although zirconia has superior mechanical properties, its opaque white color and insufficient translucency require glassy porcelain veneering on the framework to achieve a natural appearance and acceptable esthetics [ 8 ]. However, cracking or chipping of the porcelain veneer has been reported to be a major complication of these restorations [ 9 - 12 ]. The possible causes of porcelain veneer cracking are; differences in coefficient of thermal expansion (CTE) between framework and porcelain, firing shrinkage of porcelain, porosities, poor wetting of veneering, flaws on veneering, inadequate framework design to support veneer porcelain, overloading and fatique [ 8 ].

There are several solutions to overcome the veneer cracking problem due to its multifactorial nature: alternative application of techniques for veneering such as CAD/CAM produced veneer [ 13 ], modification of the firing procedures [ 14 ], and modification of the framework design [ 15 ]. Another alternative solution was to use non-veneered zirconia restorations. The translucency of zirconia was increased and full-contoured, monolithic zirconia restorations without veneering porcelain have become increasingly popular as a result of advances in CAD/CAM technology [ 8 , 16 ]. The monolithic zirconia has been used in posterior region, especially for single crowns, in order to eliminate the veneer cracking [ 17 , 18 ]. It has been suggested for use in patients with limited interocclusal space because of its ability to resist high loads with only 0.5 mm occlusal thickness [ 19 ]. The technicians can also prepare monolithic zirconia for all-on-4 prosthesis by using CAD/ CAM. Limmer et al. [ 20 ] presented 1 year results of clinical outcomes of 4 implant supported monolithic zirconia fixed dental prosthesis, and observed a few complications related to restorations. They concluded that these kinds of restorations might be a therapeutic option in the edentulous mandible.

There are 2 types monolithic zirconia materials; opaque and translucent zirconia. Opaque zirconia offers significantly greater flexural strength and indicated in the posterior regions of the mouth. Translucent zirconia has more natural esthetic properties. Lava plus high translucency zirconia (3M ESPE) has a unique shading system that gives laboratories many options for custom shading and characterization. After milling a porous green-state block, the laboratory can choose from among 18 dyeing liquids that cover the 16 Vita Classical A1- D4 shades to achieve custom coloring. The dyeing liquid is applied and then, during the sintering step, the color ions are incorporated into the zirconia. With greater strength and improved esthetics, this high translucency zirconia has the potential to be used in either the posterior or anterior regions of the mouth.

The low temperature degradation (LTD) is an aging phenomenon related to monolithic zirconia. In the presence of moisture and at low temperatures (150-400°C), slow tetragonal to monoclinic transformations occur on the surface of zirconia, then progress into the bulk of the material [ 21 ]. The growth of the transformation zone results in severe micro-cracking, grain pullout and surface roughening that leads to decrease in strength [ 22 ]. LTD was found to intensify for rougher zirconia surfaces; therefore, smooth surfaces are required to prevent LTD [ 23 ].

A definitive cementation protocol for zirconia ceramics has not been validated yet. Both the conventional and adhesive cementation techniques are feasible. For the adhesive cementation, different air-blasting protocols associated with chemical primers such as formulations containing MDP monomers or silane coupling agents are the most recommended conditioning methods for zirconia restorations, followed by dual-cured resin cements [ 24 , 25 ].

To date, many articles on monolithic zirconia have been published. However, there is still little general knowledge with regard to their mechanical behavior and reliability, and the factors that would contribute to their optimal application performance. Therefore, the purpose of this article is to give a succinct literature review on the material properties of monolithic zirconia, to summarize research articles conducted on this subject, and provide information on this alternative restoration type based on the results of original, full-length, scientific papers published in journals listed in PubMed.

Materials and Methods

A PubMed search was conducted up to May 2015. The terms of “zirconia” or “zirconium dioxide” or “yttria-stabilized tetragonal zirconia polycrystals (Y-TZP)” with “monolithic”, “full-contour”, “solid”, “translucent”, “anatomic-contoured”, “un-veneered”, “non-veneered”, “full-coverage” were used. The literature search covered all years and focused on publications that contained dental data regarding in vitro studies, case reports, clinical studies and reviews. The publications that used veneered zirconia, and the studies that did not use zirconia material as a superstructure were excluded. Full-text of the articles were obtained from different sources and the abstracts in English were used which were written in a different language instead of English.

According to PubMed search, the total number of publications that met the inclusion criteria for this review was 49. Of these, 28 were laboratory studies, 10 were case reports, 4 were clinical studies, 4 were clinical aspects and techniques, 2 were stress analyses, and 1 was a literature review article on a special subject (wear).

Most of the studies were conducted in vitro [ 17 , 18 , 26 - 51 ]. Wear properties was investigated in 19 articles [ 17 , 18 , 26 , 28 - 34 , 37 , 41 , 42 , 44 , 46 - 50 ], surface roughness in 9 articles [ 26 , 28 , 29 , 31 , 43 , 45 , 46 , 48 , 51 ], fracture strength in 6 articles [ 35 , 38 , 40 , 43 , 49 , 50 ], optical properties and color in 4 articles [ 7 , 36 , 39 , 50 ], and marginal fit in 1 article [ 27 ]. There were 2 stress analyses [ 52 , 53 ] and 4 clinical aspects and techniques [ 54 - 57 ]. There was only 1 review article about the wear behavior of monolithic zirconia against enamel [ 58 ]. Other published articles were clinical studies [ 16 , 20 , 59 , 60 ] and case reports [ 61 - 70 ].

In vitro studies

Wear: Wear means “loss of material from a surface” [ 44 ]. Wear of a material is related to several factors, such as mechanical contact, surface roughness, grain size, fracture toughness, occlusal load, temperature, chemical reactions, environment and lubrication [ 34 ]. Surface conditions is one of the most crucial factor, therefore, different kinds of surface treatments should be applied on the restorative materials in order to prevent damage of natural antagonist teeth [ 44 ].

There are two common surface treatment techniques for monolithic zirconia, such as polishing (manual/machine) or glazing (glass coating/firing) to improve the esthetic appearance of the restoration and to obtain smooth surface texture. Diamond points, rubber wheels and abrasive pastes are used in polishing procedures. Glazing is performed by firing a thin coating of glass on the surface or by firing the restoration up to temperature required for glazing [ 7 ].

The wear ability of monolithic zirconia was evaluated in 19 studies. ( Table 1 ). According to Table 1 , it can be clearly observed that polished zirconia had the lowest wear on the antagonists compared to glazed zirconia. This result was attributed to the fact that glazed zirconia loses the thin glaze after a short period of clinical function, with the result of appearance of the rough and more abrasive surface of zirconia. It was also stated that glazed layer is easily removed by chairside occlusal adjustments [ 47 ]. Only one study by Beuer et al. [ 50 ] reported higher antagonist wear with a polished zirconia than with a glazed zirconia. This difference was attributed to polishing techniques that created as smooth as or smoother than glazed surfaces in other studies. They concluded that results might be different if other polishing techniques would have been applied on zirconia surfaces.

Table 1: In vitro studies that examined the wear properties of monolithic zirconia.

Surface roughness: Preparing a smooth surface for ceramic restorations is considered as an important step because increased surface roughness associated with improper surface treatment can increase wear rate of the opposing teeth and can compromise the clinical performance of the restorations [ 71 , 72 ].

The surface roughness of monolithic zirconia was evaluated in 9 studies. Ghazal et al. [ 51 ] evaluated the effect of surface roughness of zirconia on the wear of antagonist enamel and composite resin, and found that an increase in the surface roughness significantly increased the wear of enamel and composite resin. They also reported that the maximum surface roughness of zirconia should not be greater than 0.75 μm. Alghazzawi et al. [ 43 ] found that surface roughness of polished monolithic zirconia was significantly increased with aging procedures, because the volume expansion associated with the phase transformation (tetragonal to monoclinic) during LTD leaded to grain pushout that imparted the surface roughening. Mörmann et al. [ 28 ] stated that the gloss of zirconia was slightly increased and the roughness was decreased after toothbrushing. Preis et al. [ 31 ] reported that smoother surfaces were obtained with the polished zirconia compared to ground zirconia. Hmaidouch et al. [ 45 ] investigated the effect of controlled intraoral grinding and polishing on the roughness of monolitic zirconia and compered it to veneered zirconia in their study. They reported that fewer defects and lower roughness values were obtained in monolithic zirconia compared to veneered zirconia. In addition, they found that lower roughness values were achieved after polishing compared to glazing procedure. It was showed in another study that [ 46 ], machined zirconia had higher surface roughness than glazed zirconia. Similarly, the glazed surface was found smoother than polished and ground surface [ 48 ].

However, controversial results have been obtained in other studies [ 26 , 29 ]. Janyavula et al. [ 26 ] found that the polished surfaces of monolithic zirconia were smoother than glazed surfaces. It was stated by Mitov et al. [ 29 ] that polished zirconia showed a lower surface roughness than glazed and ground zirconia. These differences may be due to the different polishing (machine or manual) and glazing (glass coating, firing) techniques, or different study protocols. It was known that machine polishing results in a significantly higher surface gloss of ceramics than manual polishing with tools for intraoral polishing [ 73 ].

Fracture strength: Fracture strength was investigated in 6 articles. In a study by Zesewitz et al. [ 35 ], zirconia showed the highest strength when luted with adhesive resin or glassionomer cements, compared to lithium disilicate and feldspathic ceramics. Similar results were obtained with Zhang et al. study [ 40 ]. In another study by Sun et al. [ 38 ], monolithic zirconia crown with a thickness of 1 mm was found equal to metal-ceramic crowns. It was also reported that strength of monolithic zirconia was higher than veneered zirconia, lithium disilicate and metal-ceramics. These results are in agreement with the study by Beuer et al. [ 50 ] that has reported monolithic zirconia had higher strength than veneered zirconia. On the contrary, the strength of monolithic and veneered zirconia was found similar in Preis et al. study [ 49 ]. Alghazzawi et al. [ 43 ] found that the strength values were not altered significantly between aged and non-aged monolithic zirconia crowns. As a result of these studies, it can suggest that monolithic zirconia that has a promising future may be an alternative to traditional veneered zirconia.

Optical properties: The creation of acceptable esthetic result with monolithic zirconia restorations is challenging because they are mono-layered restorations. Application of coloring liquids, surface characterization, glazing and polishing of zirconia are the procedures to look like natural teeth [ 36 ]. Significantly improved color adaptation to adjacent teeth is accomplished with coloring of the monolithic zirconia structures, followed by individual color characterizations achieved by surface painting. The coloring liquids with different color intensities are applied with a paintbrush prior to sintering [ 54 ].

The translucency of the monolithic zirconia restoration is also essential for optimized esthetic outcome. However, an increase in crystalline content and framework thickness in order to achieve high strength would generally result in lower translucency. Zirconia has higher contrast ratio compared to glass ceramics, and can be clinically applied with a minimum thickness of 0.4 mm [ 74 ].

There are few studies in the literature reporting optical properties and color of monolithic zirconia. In a study by Kim et al. [ 36 ] the effect of number of coloring liquid applications on color, translucency and opalescence of monolithic zirconia was investigated. The increased number of coloring liquid applications reduced the lightness and opalescence. Sari et al. [ 39 ] reported that transmission of Er:YAG laser through monolithic zirconia was lower than leucide and lithiumdisilicate reinforced glass ceramics. In another study by Kim et al. [ 7 ] it was found that polishing and glazing procedures decreased lightness, glazing increased yellowness, and increased number of coloring liquid applications made zirconia darker and more yellowish. When compared polished and glazed monolithic zirconia with veneered zirconia, it was stated that polished zirconia showed higher light translucency [ 50 ].

Marginal fit: Karl et al. [ 27 ] investigated the quality of fit of zirconia crowns and they found that monolithic zirconia showed greater passivity of fit than veneered zirconia. They showed that ceramic veneering of zirconia frameworks resulted in an increase in strain development. Monolithic contour restorations exhibited less strain.

Stress analyses studies

There are 2 studies in the literature regarding stress analyses of monolithic zirconia [ 52 , 53 ]. In the first study [ 52 ], the fracture load of zirconia was found 1.8 times greater than lithium disilicate when supported by dentin and 1.3 times greater than lithium disilicate when supported by enamel. In the second study [ 53 ] monolithic crown systems (zirconia, alumina, metal, all porcelain) were compared with the veneered crowns (zirconia, alumina, metal) in terms of compressive stress. For monolithic systems, the all porcelain showed the highest concentration of compressive stresses followed by zirconia, alumina and metal.

Clinical studies

Four articles were included in the clinical follow-up studies associated with monolithic zirconia [ 16 , 20 , 59 , 60 ]. Batson et al. [ 59 ] fabricated a total of 32 monolithic zirconia, metal ceramic and lithium disilicate posterior single crown restorations in 22 patients and evaluated them at the 6-month visit. They observed that monolithic zirconia crowns were superior in occlusion (only 20% needed adjustment) and marginal adaptation (least amount of horizontal marginal discrepancy). In another study, clinical complications and survival rates of implant supported monolithic zirconia fixed dental prosthesis in 17 edentulous patients at the 12 month visit [ 20 ]. Prosthesis survival was 88%. One of the prosthesis was fractured and the other prosthesis was removed due to the implant failure. In a clinical study by Wang et al. [ 60 ], esthetic, wear and fracture were evaluated in 35 monolithic zirconia crowns in 30 patients after 24-month visit. No fracture was found, the esthetic was satisfactory but antagonist enamel wear was observed. Stober et al. [ 16 ] evaluated the enamel wear caused by 20 monolithic zirconia crowns in 20 patients after 6 months of clinical use, and found that zirconia crowns caused greater wear of opposed enamel compared to natural teeth. Although the enamel wear was greater than natural teeth, previous studies [ 75 , 76 ] claimed that the wear is lower than or comparable with other ceramic restorations such as metal-ceramics, alumina and glassceramics. Therefore, further clinical evaluations of wear with various ceramic crown systems and over a longer time period should be conducted.

Nowadays, monolithic zirconia has become popular because of their high flexural strength, natural tooth color, less wear on the antagonists, and minimum tooth preparation [ 8 , 16 ]. For the patients with compromised occlusion or parafunction, monolithic zirconia crowns may be fabricated with as little as 0.5 mm of occlusal reduction [ 19 ]. It is possible to produce CAD/CAM-milled monolithic zirconia restorations with the new digital impression technology such as CEREC (Sirona Dental Systems) or Lava Chairside Oral Scanner (3M ESPE) [ 8 ]. The color of the restoration is homogeneous and there is no need for concern about opaque show-through during adjustment of the occlusion. It is also easy to shape and polish the material using porcelain-polishing materials [ 36 , 54 ].

Zirconia has been considered an opaque material compared to other all ceramics, but more esthetic alternative to porcelain fused to metals (PFMs) or cast gold restorations, in the areas with limited occlusal spaces [ 74 ]. The translucency of monolithic zirconia should be improved to make it a restorative option in the anterior region as well. The cementation is either adhesive or conventional [ 24 , 25 ].

This article reviewed the outcomes of laboratory and clinical studies of monolithic zirconia. The number of the articles was limited because this material has been used in a short time compared to other materials used in prosthodontics restorations. Most of the clinical studies had short follow-up periods ranging from 6 to 24 months. However, the solutions of the clinical complications of this material were not be pointed. Therefore, clinicians should be careful about the indications and limitations when making decisions regarding monolithic zirconia. According to results of the in vitro studies, it can be clearly seen that polished monolithic zirconia surfaces caused the lowest wear on the opposing teeth compared to glazed zirconia surfaces [ 58 ]. The wear is affected by the surface roughness, and machine polishing technique seems to be more successful in this manner, because the glaze layer is removed during the wear process. When considered the fracture strength of the material, it was found better than veneered zirconia [ 38 , 50 ].

Conclusions

This paper reviewed the available literature on monolithic zirconia restorations. Monolithic zirconia is emerging as a promising option. Many in vitro studies on monolithic zirconia have been published to date; however, clinical long-term evaluation is crucial and mandatory to a more thorough understanding of the mechanical behavior and reliability of these restorations. LTD in non-veneered zirconia restorations may cause severe clinical problems after several years of clinical service. As an alternative monolithic ceramic material to zirconia, lithium disilicate may be used in the clinical practice, which longer-term clinical data have been already published [ 77 - 80 ]. The authors believe that before monolithic zirconia crowns are used widely and prevalently in dental practice, studies of longer duration are necessary to validate this material. Despite the reported advantages and short-term favorable clinical reports, long-term follow-up studies of at least 10 years should be conducted. These studies will provide the much-needed data pertaining to the efficacy of zirconia material for full-contour restorations.

Conflict of Interest

The authors deny any conflict of interest.

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• DOI: 10.5005/jp-journals-10019-1449
• Corpus ID: 271079260

Evaluation of Shear Bond Strength of CAD/CAM Ultra-translucent Zirconia and Lithium Disilicate Ceramics Bonded to Enamel: An In Vitro Study

• Ibrahim A Elsisi , Omnia Nabil , Shereen A Amin
• Published in International Journal of… 29 June 2024
• Materials Science, Medicine, Engineering

44 References

Zirconia based composite scaffolds and their application in bone tissue engineering., fracture resistance of monolithic translucent zirconia crown bonded with different self-adhesive resin cement: influence of mdp-containing zirconia primer after aging, zirconium carbide for hypersonic applications, opportunities and challenges, flexural strength of cad/cam lithium-based silicate glass–ceramics: a narrative review, the influence of polishing on the mechanical properties of zirconia—a systematic review, bonding durability between zirconia and different types of tooth or implant abutments-a systematic review. part ii: outcomes of clinical studies., comparison of the shear bond strength of translucent zirconia and lithium disilicate ceramic following immediate dentin sealing, impact of different layers within a blank on mechanical properties of multi-layered zirconia ceramics before and after thermal aging., the resin bond to high-translucent zirconia-a systematic review., shear bond strength of ceramic laminate veneers to finishing surfaces with different percentages of preserved enamel under a digital guided method, related papers.

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Article Versions Notes

Nitasnoraset, K.; Riddhabhaya, A.; Sessirisombat, C.; Hotokezaka, H.; Yoshida, N.; Sirisoontorn, I. Shear Bond Strength of Clear Aligner Attachment Using 4-META/MMA-TBB Resin Cement on Glazed Monolithic Zirconia. Polymers 2024 , 16 , 1988. https://doi.org/10.3390/polym16141988

Nitasnoraset K, Riddhabhaya A, Sessirisombat C, Hotokezaka H, Yoshida N, Sirisoontorn I. Shear Bond Strength of Clear Aligner Attachment Using 4-META/MMA-TBB Resin Cement on Glazed Monolithic Zirconia. Polymers . 2024; 16(14):1988. https://doi.org/10.3390/polym16141988

Nitasnoraset, Kasidit, Apiwat Riddhabhaya, Chidchanok Sessirisombat, Hitoshi Hotokezaka, Noriaki Yoshida, and Irin Sirisoontorn. 2024. "Shear Bond Strength of Clear Aligner Attachment Using 4-META/MMA-TBB Resin Cement on Glazed Monolithic Zirconia" Polymers 16, no. 14: 1988. https://doi.org/10.3390/polym16141988

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