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Fracture Resistance of Zirconia Fixed Partial Dentures.

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Abstract

Over the past decades, attention in more esthetically pleasing and metal free restoration has amplified for all ceramic restorations. Additionally, numerous systems are provided to employ sophisticated CAD/CAM technology. After the innovation in zirconia material, new sight of lights provided for designing fixed all-ceramic partial dentures without limitation of the fixed partial denture' size. Therefore, the aim of the current research was to determine fracture resistance of zirconia FPDs using the PubMed and Medline database English literature by the terms: "Fracture resistance", "Zirconia", "Fixed partial dentures" and "framework design" and "occlusal forces".
ABR Vol 8 [3] May 2017 205 | P a g e ©2017 Society of Education, India
ADVANCES IN BIORESEARCH
Adv. Biores., Vol 8 [3] May 2017: 205-208
©2017 Society of Education, India
Print ISSN 0976-4585; Online ISSN 2277-1573
Journal’s URL:http://www.soeagra.com/abr.html
CODEN: ABRDC3
DOI: 10.15515/abr.0976-4585.8.3.205208
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Fracture Resistance of Zirconia Fixed Partial Dentures
Vahideh Nazaria, Mohammed Hussein Alsharbatya, Moeen Hosseini Shirazia*
aPost-graduate Student, Department of Prosthodontics, School of Dentistry, Tehran University of Medical
Sciences, Tehran, Iran.
*Corresponding author: Moeen Hosseini Shirazi
Department of Prosthodontics, School of Dentistry, Tehran University of Medical Sciences, Tehran, Iran,
North Karegar st, Tehran, , Iran. Postal code: 14399-55991
E-mail: Moeen.hshirazi@yahoo.com, Tel: +989113554859, Fax: +982188015801
ABSTRACT
Over the past decades, attention in more esthetically pleasing and metal free restoration has amplified for all ceramic
restorations. Additionally, numerous systems are provided to employ sophisticated CAD/CAM technology. After the
innovation in zirconia material, new sight of lights provided for designing fixed all-ceramic partial dentures without
limitation of the fixed partial denture’ size. Therefore, the aim of the current research was to determine fracture
resistance of zirconia FPDs using the PubMed and Medline database English literature by the terms: “Fracture
resistance”, “Zirconia”, “Fixed partial dentures” and “framework design” and “occlusal forces”.
Keywords: Zirconia, CAD/CAM, FPDs
Received 19.03.2017 Revised 04.04.2017 Accepted 29.04.2017
How to cite this article:
V Nazari, M H Alsharbaty, M H Shirazi.. Fracture Resistance of Zirconia Fixed Partial Dentures. Adv. Biores., Vol 8 [3]
May 2017.205-208
INTRODUCTION
Metal-ceramic restorations represent the desirable and efficacious treatment modality. Clinical trials have
shown that metal-ceramic FPDs have ability of long lasting [1]. Nonetheless, metal frameworks have
inherent disadvantages include material corrosion and discoloration of gingival tissues adjacent to the
crown margins [2]. In comparison to the metals, dental ceramics has enhanced biocompatibility and
improved esthetics (3). Capability of transmission of light through the ceramic mass provides such a
promising property to it. Additionally, dental ceramics exhibit lessened plaque accumulation, low thermal
conductivity, resistance to corrosion and color stability. Inadequate mechanical strength and stability
caused by brittleness as well as low tensile strength of these materials limit their range of indication [3].
Zirconia copings, which are fabricated by computer-aided design/computer-aided manufacturing
(CAD/CAM) has been developed accurately with high mechanical strength [4].
Yttria-tetragonal-zirconia-polycrystal (Y-TZP) was introduced in 2002, as a framework material. The Y-
TZP indicated for all-ceramic FDPs there are growing interest for other clinical applications [5]. Because
of high strength and fracture toughness associated with favorable optical and bio computability, Y-TZP
considered being a worthy alternative to metal in FPDs [6]. Mechanical property of Y-TZP is higher
compared to all other ceramics for dental use. It is reported that Y-TZP zirconia has fracture toughness of
6–10MPa, a flexural strength of 900–1200 MPa and a compression resistance of 2000 MPa [7]. The high
strength of the Y-TZP was attributed to a phase transformation toughening mechanism. This involves the
transformation of metastable tetragonal grains (t) to the monoclinic phase (m) under stress at the crack
tip, which, accompanied by volume expansion, induces compressive stresses that counteract crack
propagation [8]. At the edge of the crack, the compressive stress transformed tetragonal grains
counteracts the tensile stress field and stimulates the propagation of the crack and finally inhibits further
propagating of the crack [9].
Two major techniques introduced for fabrication of Y-TZP frameworks. In the first method milling
enlarged frameworks out of homogenous blanks, which are typically carried out by a nonsintered (green
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body) or in different presintered stages. Then milled frameworks sintered and shrunk to the desired
dimensions [10-13]. The second technique contains milling the frameworks directly with the final
dimensions out of highly dense sintered prefabricated blanks [10-12, 14].
In clinical research on the 3 to 5-unit FDPs revealed short-term (2-3 years) follow-up of zirconia-based
posterior FDPs. It is reported the survival rates of this system is higher than 85 percent [15-17].
This review article describes the fracture resistance of zirconia-based fixed restorations, including results
of current in vitro studies and the clinical performance of these restorations. An electronic search has
been conducted, during 2016, via PubMed and Medline database English literature. Peer-reviewed
articles were targeted following key-words have been used: Fracture resistance”, “Zirconia”, “Fixed
partial dentures” and “framework design and occlusal forces. Available full-text articles were read.
Related articles were also scrutinized. Hand search was also driven.
STUDIES ON FRACTURE RESISTANCE OF ZIRCONIA FPDs
To our knowledge, frequent researches were done to determine the fracture resistance of ZrO2-based
FPDs [13, 18-20]. Because of the mechanical properties of zirconia, it is possible to use in posterior FPDs
and permit substantial reduction in core thickness (11). Mechanical strength of zirconia frameworks is 3
times greater than other all ceram. SO, it is able to withstand physiological occlusal forces applied in the
posterior region [21-24]. However, the cracking is still as a major disadvantage of zirconia restorations.
For instance, the Porcelain chipping of the zirconia restorations rates up to 20% in 5-years period [23,
25].
Based on the in vitro studies that stresses induced in the cervical portion of an all-ceramic FPD supported
by chamfer preparations during loading and might affect the connector areas. In this area the stress
concentration can reach critical levels and terminate to the FPD to fracture. So, it is suggested that all-
ceramic FPDs cemented non-adhesively should be supported by shoulder preparations to resist extensive
loading whenever it may be expected [26].
Failure mode of the zirconia FPDs
According to the in vitro and in vivo studies, the highest failures were found in the connector area in all-
ceramic FDPs. It propagated obliquely, connecting the gingival embrasure of the connector and the
occlusal loading point on the pontic [24, 27-31]. The line of crack propagation relates to the stress
localization. According to the prior finite element analysis and photoelastic reports [27, 28] the gingival
surface of both connectors is initially in highest tension, whereas the occlusal surface of the abutment
side of the gingival connector, as well as the loading point on the pontic, are in maximum compression.
Ceramic materials enable to withstand to occlusal forces better than tensile forces and leads to the
fracture the restorations.
Effect of the framework design
The framework design can affect Fracture strength [32, 33]. The effects of framework design on fracture
strength of zirconia restorations are fully studied [5, 29, 34-37]. Fracture load for zirconia-based all
ceramic restorations with anatomically designed frameworks was higher than uniform thickness
frameworks [4, 32]. It is claimed the resistance might related to the thinner layer of layering material and
adequate support of layer by the supported zirconia framework at the occlusal surface [32, 33].
Furthermore, the fracture resistance of the zirconia posterior FPDs affected by the connector size and
shape [5, 19]. Smaller cross sectional area of connectors, the needed load for fracture of core will be
decreased [19]. In this regard, Plengsombut et al, shown that higher maximum failure loads were found
for the round connector design compared to the sharp connector design for ZirCAD and CAD (5).
Furthermore, Sundh et al, reported that mean fracture resistance of groups with 4×4 mm connector was
higher than 3 ×3 mm connector [19].
Potential to withstand occlusal forces
Dental restorations are subjected to intermittent occlusal forces during mastication and swallowing. The
chewing frequency and the places of maximal force on the occlusal surface were relatively constant [38].
The forces exerted on the occlusal surface seldom exceeded 10 to 15 pounds in normal chewing [38].
Also, the maximum bite forces that the stomatognathic system can exert in the posterior area (300 to
880N) for the first molar [39, 40]. Some bruxers and clenchers present bite forces six-times higher than
that of non-bruxers [40], while Patients with symptoms of dysfunction of the masticatory system show
lower bite forces that increase as the symptoms disappear (41) Additionally there are difference for
biting forces among male and female where 807N for males and 650N for females in the molar region
[42]. The biting force decrease with increasing age, because of the age-dependent deterioration of the
dentition [41]. Zirconia FPDs have the vital role to withstand occlusal forces applied in the posterior
region. So, represent interesting alternatives to replace metal-ceramic restorations [22, 43].
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CONCLUSION
Today, all-ceramic restorations are becoming more popular because of material improvements and
advances in fabrication technologies. CAD/CAM technology, it is possible now to use new high-strength
ceramic materials to fabricate all-ceramic restorations with higher and uniform material quality,
standardized manufacturing processes and reduced production costs. Limited number of short-term
clinical studies on posterior Y-TZP-based FPDs showed success rates of 100% with observation periods
between 2-3 years. Further assessments are needed before recommending these restorations for daily
practice [1].
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This paper continues a study of the literature on masticatory function. In this section, a review is presented of data on the speed of movement of the mandible, the rate of chewing, and the forces developed in chewing.
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The aim of this study was to evaluate the effect of different types and design of zirconia frameworks as well as the effect of different veneering ceramics on the fracture strength of crowns. The importance of different abutment materials was also evaluated. Eighty cores, 40 in a fully-sintered zirconia material and 40 in a pre-sintered zirconia material were made. Twenty cores of each material were made with a core of even thickness shape (ES) and 20 were made with a core with anatomical shape (AS). The cores were divided into subgroups and veneered with one of two different veneering ceramics: a porcelain and a glass-ceramic material. In total eight groups of 10 crowns were made. They were all cemented onto abutments made of resin. One extra group of 10 AS, pre-sintered zirconia cores veneered with glass-ceramic were made and cemented onto titanium abutments. All crowns underwent thermocycling and mechanical pre-load and were finally loaded until fracture. AS crowns withstood significantly higher loads than ES crowns (P-value <0.001), and crowns with titanium abutments withstood significantly higher loads than crowns supported by abutments made of inlay pattern resin (P-value <0.001). Three types of fracture were noted: minor and major fracture of the veneering ceramic, and complete fracture through core and veneer. ES crowns showed significantly more major fractures of the veneering ceramic than AS crowns. This in vitro study indicates that the design of the core, as well as the abutment support, significantly influences fracture load and fracture mode of yttria-stabilized tetragonal zirconia polycrystals crowns.
Article
This study investigated the influence of zirconia coping designs on the fracture load of all-ceramic crown. Four kinds of zirconia copings were designed (a: Conventional zirconia coping with flat occlusal surface: thickness of the each coping is 0.6 mm evenly, and at the cervical margin area, the coping is adjusted sharply so as to fit preparation margin, b: Conventional zirconia coping with shoulder collar of 1 mm: thickness of the each coping is 0.6 mm evenly, and there is a collar of 0.6 mm from the margin, c: Zirconia coping with following original cuspal configuration (concave): two inclined cusp planes, and at the cervical margin area, the coping is adjusted sharply so as to fit preparation margin, and d: Zirconia coping with supporting configuration on the occlusal area: supporting configuration against the occlusal force, and at the cervical margin area, the coping is adjusted sharply so as to fit preparation margin) and porcelain was fired. Vertical and lateral load were conducted until fracture. Coping design affected the fracture load; conventional uniform thickness coping design showed the lowest load (a), whereas cuspal configuration to perform even thickness of porcelain showed the highest fracture load both load directions (c).
Article
Most all-ceramic fixed dental prostheses (FDPs) fail at the connectors. The purpose of this study was to determine the effect of 2 connector designs on the fracture resistance of core materials used for all-ceramic FDPs. Three materials were tested: (1) heat-pressed lithium disilicate glass ceramic (IPS e.max Press (Press)), (2) milled lithium disilicate glass ceramic (IPS e.max CAD (CAD)), and (3) milled yttrium-stabilized tetragonal zirconia polycrystals (Y-TZP) (IPS e.max ZirCAD (ZirCAD)). Specimens were made into 30 x 4 x 4-mm bars to represent 3-unit FDPs. Two connector designs, round (0.60 +/-0.01-mm radius of curvature) and sharp (0.06 +/-0.001-mm radius of curvature), with a 3.00 +/-0.05-mm cross-section for each connector, were studied (n=5). Each specimen was loaded to fracture in a universal testing machine with a crosshead speed of 0.1 mm/min. Data were analyzed with a 2-way univariate ANOVA and Tukey HSD test (alpha=.05). Mean (SD) failure loads for round connector designs were 684.2 (70.1) N for ZirCAD, 260 (7.8) N for CAD, and 172.9 (35.5) N for Press. Mean (SD) failure loads for sharp connector designs were 386.3 (51.5) N for ZirCAD, 87.9 (7.0) N for CAD, and 125.1 (15.1) N for Press. The 2-way univariate ANOVA indicated statistically significant differences (P<.005) for material and connector design, and, also, a significant interaction between material and connector design. Higher maximum failure loads were found for the round connector design when compared to the sharp connector design, for ZirCAD and CAD. However, this difference was not statistically significant for the Press groups. SEM subjective assessment of the fractured specimens revealed that the fracture initiated from the gingival surface (tensile) of the connector toward the pontic (central loading point). Fracture resistance of ceramic core materials is affected by fabrication technique and connector design. Connector design affected fracture resistance of the milled ceramic, but not the pressed ceramic.