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 Table of Contents  
ORIGINAL ARTICLE
Year : 2018  |  Volume : 15  |  Issue : 1  |  Page : 7-13

Flexural strength of four-unit implant-supported ceramic veneered zircon and full zircon fixed dental prosthesis


1 Ministry of Health, Faculty of Dentistry, Tanta University, Tanta, Egypt
2 Department of Fixed Prosthodontics, Faculty of Dentistry, Tanta University, Tanta, Egypt

Date of Submission03-Jun-2017
Date of Acceptance03-Aug-2017
Date of Web Publication4-Apr-2018

Correspondence Address:
Ayatalla A Bedair
Ministry of Health, Tanta
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/tdj.tdj_29_17

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  Abstract 

Purpose
To investigate the flexural strength and the fracture loads of bilayered and monolithic zirconia fixed dental prostheses (FDPs).
Materials and methods
For flexural strength test, two groups of bar-shaped (25 × 5 × 2 mm3) monolithic zirconia (n = 5) and bilayered zirconia–porcelain (n = 5) specimens were prepared using computer-aided design and manufacture system for zirconia and hand layering for porcelain. Flexural strength test was performed using universal testing machine at cross-head speed of 1 mm/min. For fracture resistance test, 20 master models composed of two implant analogues and their corresponding titanium implant abutments embedded vertically in autopolymerizing acrylic resin blocks to mimic clinical conditions for a four-unit FDPs. Twenty FDPs were fabricated and divided into two groups (n = 10): group A, bilayered zirconia four-unit implant-supported FDPs; group B, monolithic zirconia four-unit implant-supported FDPs. Samples in each group were cemented to their corresponding model and then underwent artificial ageing by thermocycling of 5000/cycle. The fracture resistance test was done using universal testing machine at cross-head speed of 1 mm/min. Statistical analysis of the results and comparison between each two groups were performed using independent t-test (significance: P ≤ 0.05).
Results
Monolithic zirconia group recorded statistically significant higher flexural strength and fracture resistance compared to bilayered group.
Conclusion
Strength of monolithic zirconia is significantly higher than bilayered zirconia.

Keywords: bilayered zirconia, flexural load, fracture resistance, implant.supported prosthesis, monolithic zirconia


How to cite this article:
Bedair AA, Korsel AM, Elshahawy WM. Flexural strength of four-unit implant-supported ceramic veneered zircon and full zircon fixed dental prosthesis. Tanta Dent J 2018;15:7-13

How to cite this URL:
Bedair AA, Korsel AM, Elshahawy WM. Flexural strength of four-unit implant-supported ceramic veneered zircon and full zircon fixed dental prosthesis. Tanta Dent J [serial online] 2018 [cited 2018 Aug 21];15:7-13. Available from: http://www.tmj.eg.net/text.asp?2018/15/1/7/229243


  Introduction Top


In the last decades, since the development of porcelain fused to metal restorations in the early 60s, they have represented the 'gold standard' for years in prosthetic dentistry, due to their good mechanical properties and to somewhat satisfactory esthetic results, along with a clinically acceptable quality of their marginal and internal adaptation [1]. Nevertheless, the technical procedures of investing wax patterns and casting precious metal alloys involve many technical variables and a considerable number of operative steps and firing cycles, making the final quality of the restorations highly technique-sensitive. Moreover, the metal framework and the layer of opaque porcelain needed for masking the under laying metal grayish shade are likely to introduce a significant limitation for the esthetic result due to the absence of translucency, especially when a clear tooth color is to be reproduced. In fact, metal–ceramic restorations can only absorb or reflect light, while dental tissues show a high degree of translucency [2].

With the aim of replacing the infrastructure of metallic dental prostheses, structural ceramics have been improved and have become increasingly more popular in dentistry. Patients are aware that ceramic materials are able to mimic the appearance of natural teeth and over the past several decades this has driven vast improvements in ceramic dentistry. However, improvements in the esthetics of dental restorations cannot compromise their strength to withstand maximum bite forces [3].

An ideal all-ceramic dental material should exhibit excellent esthetic characteristics, like translucency, natural tooth color, outstanding light transmission and, at the same time, optimal mechanical properties, like flexural strength, fracture toughness and limited crack propagation at the functional and parafunctional load conditions, in order to ensure long-term service. Wide varieties of ceramic systems have been developed to match the growing application of ceramic restorations. This has led to an extensive and overlapping classification of porcelain according to its composition, application, manufacture and processing as well as substructure material.

Among the dental ceramics, zirconia has emerged as a versatile and promising material because of its biological, mechanical and optical properties, which has certainly accelerated its routine use in computer-aided design and manufacture (CAD/CAM) technology for different types of prosthetic treatment. Laboratory and clinical studies have shown substantially increased flexural strength and fracture toughness for zirconia fixed dental prostheses (FDPs) compared with other ceramic materials and reported prosthetic survival and success rates comparable with conventional porcelain fused to metal FDPs [4].

Zircon-based all-ceramics are currently used to fabricate copings and implant abutments and for partial and complete arch frame-works on both natural teeth and implants, in both anterior and posterior oral cavity areas [5]. The lack of translucency of high-density polycrystalline zirconium-oxide ceramic makes the use of veneering porcelain with inferior material properties necessary to achieve an aesthetic outcome [6]. Although zirconia fracture is rarely seen, the risk of veneer ceramic fracture is rather high. Reducing such risk has therefore become an important issue for manufacturers. Following the development of dental material science, the translucency of zircon has been improved. By internal and external stain techniques, full-contour (monolithic) zircon restorations can now be used. Nevertheless, there is still a significant lack of medium to long-term data on zircon-based implant-supported restorations, and most clinical studies have investigated single-crown restorations and FPDs supported mainly by natural teeth [7],[8].


  Materials and Methods Top


Approval for this research was taken according to ethics committee of Faculty of Dentistry, Tanta University. Bar and bridge-shaped specimens for flexural strength and fracture resistance tests, respectively, were prepared.

Preparation of bridge specimens

Manufacture of master models

Two cement-retained titanium abutments of premolar and molar were screwed into their corresponding implant analogues using a hex driver. Sample assembly was mounted vertically inside metal mold with the aid of the dental surveyor in which a self-cure acrylic resin was poured to mimic clinical conditions for a four-unit FDP. The distance between the apexes of the two analogues in the autopolymerized acrylic resin block was 23 mm, corresponding to the average clearance between a first premolar and a second molar.

Manufacture of the fixed dental prostheses

Twenty four-unit FDPs were divided into two groups (n = 10):

  1. Group A (monolithic zirconia FDPs):


  2. A commercial dental CAD/CAM system was used to fabricate monolithic zirconia FDPs. The contour of the bridges was unified to a thickness of 2 mm at the central fossa, 2.5 mm at the height of contour and 1 mm at the cervical margin. The connector's dimensions were 4 mm occlusogingival height and 4 mm buccolingual width (4 × 4 m 2).

  3. Group B (ceramic veneered zircon FDPs):


  4. Ten zirconium-oxide frame-works were fabricated using CAD/CAM system. Minimum thickness of the frame-works at the occlusal and lateral circumferences was 0.8 mm and at the cervical margin a thickness of 0.5 mm was established. Connectors' dimensions were 4 × 4 m 2. Air-particle abrasion was performed on the external surface of all frame-works with 100 μg Al2O3 at a distance of 10 mm. For obtaining standardized veneer application, one sample of group A was chosen and from which a silicone template was prepared. The putty impression was sliced into sections: mesiodistal and buccolingual sections that had a perfect fit on the bridge surfaces. The indexes were placed on the framework so the gap between the framework and the index was used as a guide for veneer application to establish standardized contours and thickness. Porcelain was layered on the copings. The hand-veneering procedure involved two firing cycles, the first was at 910°C and the second at 900°C according to manufacturer's information. After that, all specimens were glazed at 890 and 850°C. FDPs were cemented to the abutments with temporary luting cement. A load of 30 N was applied to the occlusal surface of the specimens during cement setting period using cementing device and a universal testing machine to control cement thickness.


Specimens were undergone artificial aging, by thermocycling for 5000 cycles, which corresponds to simulation of ~6 months of function.

Preparation of bar specimens

Ten bar-shaped specimens were divided into two groups (n = 5):

Group A (monolithic zirconia)

Bar-shaped specimens (length: 25 mm, width: 5 mm, height: 2 mm) were prepared from zirconia blocks using CAD/CAM system. After they have been milled, they sintered to full density in a special furnace at 1340°C for 90 min then glazed at 890 and 850°C.

Group B (bilayered zirconia–porcelain)

Bar-shaped specimens (length: 25 mm, width: 5 mm, height: 0.8 mm) were CAD/CAM prepared from zirconia blocks. One side of each specimen was blasted with 100 μg Al2O3. Subsequently, the veneering ceramic was layered as a bar (25 × 5 × 1.2 m 3) on the 0.8-mm-thick zircon bar. The hand-veneering procedure involved two firing cycles, the first was at 910°C and the second at 900°C.

Fracture resistance test

Bridge samples were secured to the lower fixed compartment of a universal testing machine and subjected to a vertical compression load. The load was applied occlusally at the connector area between the two pontics using a metallic rod with spherical tip (7.9 mm diameter) attached to the upper movable compartment of testing machine and traveling at cross-head speed of 1 mm/min. Tin foil sheet was placed in-between to achieve homogenous stress distribution. The load at failure was manifested by an audible crack and confirmed by a sharp drop at load–deflection curve recorded using computer software. The load required to fracture was recorded in Newton [Figure 1].
Figure 1: Bridge specimen ready for force insertion.

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Flexural strength test

Flexural strength test was done by using the universal testing machine. The bar specimens were placed flat on the lower fixed compartment of testing machine that had supporting rods with a test span equal to 22 mm. A vertical compression load was applied at the center of the specimen at 1.0 mm/min cross-head speed.

In bilayered specimens, the load was applied on the veneer surface; that is, the veneer layer is subjected to compressive stress in consideration of the clinical situation [Figure 2].
Figure 2: Bar- shaped specimen ready for force insertion.

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Statistical analysis

Data analysis was performed in several steps. Initially, descriptive statistics for each group results. Independent t-test was done to detect significance between both groups in both tests. Statistical analysis was performed using SPSS software (version 17; SPSS Inc., Chicago, Illinois, USA).


  Results Top


It was found that, monolithic zirconia group recorded higher fracture resistance load than veneered zirconia group [Table 1]. The difference between both groups was statistically significant as indicated by independent t-test (t = 2.641, P = 0.0247) [Table 2].
Table 1: Descriptive statistics of fracture resistance results for both experimental groups

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Table 2: Comparison of fracture resistance results between both experimental groups

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The fracture mode for the monolithic bridge specimens was a bulk fracture at the connector area between the two pontics [Figure 3]. Whereas bilayer bridge specimens exhibited mixed cohesive failure of the frame-works and layering materials. Complete fracture of bilayered group (fracture of both the layering material and the zirconia coping) occurred also at the connector area between the two pontics. Only one specimen exhibited a complete fracture at the mesial wall of the premolar retainer [Figure 4].
Figure 3: Fracture mode of the monolithic specimens; the arrow pointed to a bulk fracture at the connector area between the two pontics.

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Figure 4: The arrow pointed to Chipping of the veneering layer, the circle present bulk fracture at the mesial wall of the premolar retainer.

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Cracks and chipping of the veneering porcelain were observed at loads that were lower than the load at complete fracture (ranged between minimum value 790.81 N and maximum value 810.72 N).

Regarding flexural strength test, it was found that, monolithic zirconia group recorded higher flexural strength than bilayered zirconia group [Table 3]. The difference between both groups was statistically significant as indicated by independent t-test (t = 4.847, P = 0.001) [Table 4].
Table 3: Descriptive statistics of flexural strength results for both experimental groups

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Table 4: Comparison of flexural strength results between both experimental groups

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In monolithic group, all specimens were broken into two pieces [Figure 5], while in bilayered group, delamination of porcelain layer was observed in all specimens before total fracture of both layers [Figure 6].
Figure 5: Fracture mode of monolithic zirconia specimen.

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Figure 6: Fracture mode of bilayered zirconia–porcelain.

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  Discussion Top


It has been implied that stress intensity in the prosthesis is proportional to the length of the bridge [9]. Therefore, long restorations are expected to experience higher tensile stresses which are further destructive for ceramics specially when used in high stress areas such as posterior area. That is why the present study focused in long span (four-unit) posterior bridges. Regarding the flexural strength test, the test span was adjusted to be nearly the same length of the bridge specimens.

The special, time-dependent behavior of zirconia restorations after placed in situ makes it necessary to simulate the effect of aging in the present study by thermocycling as the continuous alternations of mouth temperature caused by breathing as well as cold and hot food leads to tensions in the material mass and subcritical crack [10].

Previous studies that evaluated the clinical performance of zirconia-based restorations demonstrated that fracture of the veneering ceramic was the most frequent clinical problem with zirconia restorations for both tooth-based and implant-based restorations [11],[12]. It was concluded that the veneering porcelain is the weakest link, and improving its strength could reduce the incidence of veneering porcelain chipping [13].

Various approaches have been introduced to overcome chipping of zirconia-based restorations, including application of monolithic zirconia restorations [14]. Monolithic zirconia prostheses have been proposed where the prosthesis is entirely fabricated from zirconia. Such treatment modality appears to be promising for tooth-supported prostheses [14] and implant-supported prostheses [15] especially with the increased application of stained or translucent zirconia that overcomes the problem of high value of zirconia [16],[17]. However, there is a lack of scientific information regarding whether or not monolithic zirconia restorations can function with sufficient durability.

In the present study, the statistically significant higher strength of monolithic group compared to the bilayered one in both flexural strength and fracture resistance tests indicated that the strength depends on the thickness of zirconia. These results are in agreement with Matsuzaki et al. [18] who observed that the strength (flexural strength and fracture resistance) of both Zpex (translucent zircon) and TZ3YB (conventional opaque zircon) decreased as the thickness of zirconia core material decreased compared to layering porcelain thickness, and the higher values were found in monolithic zirconia compared to bilayered zirconia–porcelain specimens with the same thickness.

A study 'analyzing the fracture loads of all-ceramic monolithic crowns' also indicated a superior performance for the monolithic design. Even though the monolithic system was made of lithium disilicate, better results were obtained when compared to bilayered zirconia [19].

Beuer et al. [20] evaluated the load-bearing capacity of four different zirconia-based crowns, including zirconia core with veneer layer produced either by powder build-up or CAD/CAM technique, glazed monolithic zirconia, and polished monolithic zirconia. The results showed that zirconia in bilayered configuration had significantly lower load-bearing capacity than the other crowns' design. This result confirms the results of the present study.

Recently, a number of studies have been found to compare between fracture loads of layered and monolithic zirconia restorations. De Kok et al. [21] in a study investigating mechanical performance of implant-supported posterior crowns, found also that the fracture loads were greater for monolithic zirconia crowns than for conventional layered zirconia-based crowns.

Lameira et al. [22] evaluated the effect of design and surface finishing on the fracture resistance of zirconia crowns in monolithic and bilayered configuration after artificial aging. It was found that monolithic zirconia crowns present higher fracture loads than bilayered zirconia.

Several authors investigated fracture loads of screw-retained zirconia-based molar restorations fabricated with different restorative materials and designs. Four groups of zirconia-based molar restorations were tested; porcelain-layered zirconia-based restorations, indirect composite-layered zirconia-based restorations, metal–ceramic restorations, and monolithic zirconia restorations. It was found that fracture loads were significantly higher for monolithic zirconia restorations than for bilayered restorations [23].

Oilo et al. [24] compared the fracture resistance of monolithic, anatomic contour zirconia crowns with bilayered crowns with and without a cervical zirconia collar. He concluded that monolithic, anatomical contour design gave higher fracture resistance than traditional core veneer design and crowns with a cervical zirconia collar had higher fracture resistance than the core veneer design, but also lower than the monolithic crowns.

All those previous studies in different situations were comparing between bilayered and monolithic zirconia restorations and results are in agreement with the present results but they focused in single-crown restorations while the present study focused in long span (four-unit) bridges.

In the present study, the fracture resistance test showed that the mode of fracture in monolithic group represented a bulk fracture at the connector area between the two pontics. In bilayered group, complete fracture (fracture of the veneering layer and the zirconia core) also occurred in the connector area between the two pontics. Investigations using finite element analysis showed that maximum tensile stresses are concentrated in the connector area [25]. Both in-vitro and in-vivo studies demonstrated that fracture of the connectors was the exclusive mode of failure in all-ceramic FDPs [26],[27]. Kamposiora et al. [9]explained the phenomenon as follows: thin and irregularly shaped parts of the framework, such as the connector area, reach critical strain earlier than thicker parts, such as the pontics and the abutments, while loading. Thus, FDPs are expected to fail in these more easily bending areas. Therefore, connector dimensions are crucial for fracture resistance [28].

Cracking and chipping of the veneering porcelain in bilayered bridges were observed before complete fracture had occurred. This may be due to mechanically defective microstructural regions in the porcelain, areas of porosities [29], surface defects or improper support by the framework [30], low fracture toughness of the veneering porcelain [31] and finally overloading and fatigue [32].

In the present study the lowest fracture load was recorded (997.34 N) for the bilayered group and (1314.7 N) for monolithic group; still higher than the maximum chewing forces reported in the literature, which have been reported to range between 600 and 800 N in the molar region for healthy young adults [33],[34]. Therefore, the results indicated that the fracture loads presented by both groups may tolerate the clinical applications without restrictions. Since the examined FDPs were four-unit – that is, complex – and still survived high loading forces, it can be mentioned that long-spanned zirconia-based FDPs appear to be a promising treatment method even for posterior areas.

Nevertheless, in case of bilayered group the results of fracture resistance test were recorded at complete fracture but cracking or chipping of the veneering layer occurred at lower load that ranged between 790.81 and 810.72 N. Consequently, bilayered zirconia-based restorations are vulnerable to failure inside the mouth especially in high stress areas.


  Conclusion Top


From the current findings it was concluded that:

  1. Flexural strength and fracture resistance were significantly higher for monolithic zirconia restorations than for bilayered zirconia restorations.
  2. Monolithic zirconia FDPs are well-suited for clinical use in areas of high stress and long span restorations compared to bilayered FDPs.


Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

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