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EGYPTIANDENTAL JOURNAL

Vol. 61, 1757:1768, April, 2015

*Associate Lecturer of Fixed Prosthodontics, Faculty of Oral and Dental Medicine, Cairo University,Cairo, Egypt** Professor of Fixed Prosthodontics, Faculty of Oral and Dental Medicine, Cairo University, Cairo, Egypt*** Professor of Fixed Prosthodontics, Faculty of Oral and Dental Medicine, Cairo University, Cairo, Egypt Professor of Fixed Prosthodontics, Faculty of Dentistry, King Abdulaziz University, Saudi Arabia

FRACTURE RESISTANCE OF DIFFERENT YTTRIUM ZIRCONIA POSTERIOR FRAMEWORK DESIGNS

Ahmed Idris*; Ashraf Elkaraksy** and Hanan Naguib***

ABSTRACTStatement of the problem: Removal of limited amount of sound tooth structure, including

axial reduction and the finish line configuration may affect fracture resistance of posterior bridges.

Objective: This study evaluated the effects of different axial coping and finish line thicknesses, as well as connector designs on the fracture resistance of yettrium zirconia posterior fixed partial dentures framework

Methods: CAD/CAM Cerec-inLab system was used to mill thirty bridge frameworks which are divided into three groups (each of 10 frameworks) according to finish line and coping thicknesses: Group 1: Chamfer finish line thickness 0.3mm, coping thickness 0.3mm.Group 2:Chamfer finish line thickness 0.3mm, coping thickness 0.5mm. Group 3: Chamfer finish line thickness 0.5mm, coping thickness 0.5mm. Each group was further subdivided into 2 equal subgroups according to the design of the connector area: Subgroup A: (5 samples) The connector is directed occlusaly. Subgroup B: (5 samples) The connector is straight. 2 stainless steel models were fabricated. Each model resembled a prepared lower second premolar and lower second molar with an edentulous span area for missing lower first molar. The first model is milled with 0.3mm chamfer finish lines for both abutment, while the second model is milled with 0.5 mm chamfer finish line for both abutment. Both models are duplicated and poured with special Cerec stone recommended by the manufacturer. Then the models are further duplicated and poured with epoxy resin. The Cerec stone models were scanned by the Cerec in-lab system, then frameworks designs were adjusted regarding the axial wall thickness and connector thickness. The frameworks were milled and sintered in the zyrcomate furnace. Then the veneering procedure was carried out utilizing press on zirconia technique. Wax pattern resembling the veneering were designed and milled utilizing CAD/CAM technology then presses using e.max zirpress ingots. Then the framework were cemented on the epoxy resin dies using resin cement.Finally the frameworks were fractured using universal testing machine to detect the fracture resistance.

Results: The statistically significantly highest mean fracture resistance was found with (Upward connector with 0.5 mm finish line and 0.5 mm coping) 2307.3N. Lower mean fracture

(1758) Ahmed Idris, et al.E.D.J. Vol. 61, No. 2

INTRODUCTION

During the past 40 years, porcelain fused to metal technique has been extensively used in fixed partial dentures. However; the public scare about allergic adverse side effects of dental alloys and inherent metal opacity has accelerated the development of alternatives to metallic dental restoration, therefore numerous attempt have been made to develop all ceramic systems which eliminate metal infrastructure. (1, 2)

However; dental ceramics are brittle and their low fracture resistance and relatively low flexural strength still limit the possibility of manufacturing FPDs using all-ceramic frameworks. Recently, yttrium oxide partially-stabilized zirconia (Y-TZP), has been made available to dentistry through the CAD/CAM-technique. Yttrium-Zirconia ceramics have been shown to have excellent mechanical performance, and superior strength and fracture resistance compared to other ceramics. Since Y-TZP has attractive mechanical properties, it could be of interest in the manufacturing of all-ceramic bridges intended for placement in premolar and molar regions. (1)

The idea of using CAD/CAM techniques for the fabrication of tooth restorations was originated with Duret in the 1970s. Ten years later, Mormann developed the CEREC-system, first marketed by Siemens, which enable the first chair side fabrication of restorations with this technology. There has been a marked acceleration in the development of other

CAD/CAM laboratory systems in recent years until reaching the Cerec 3 system, and finally the Cerec-inLab system, which is designed for indirect fabrication of all ceramic restorations. (3)

Currently, dental prosthetic treatment follow principles based on conserving sound tissue, con-cerning the removal of limited amount of sound tooth structure, including axial reduction and the fin-ish line configuration. Modern adhesive technology and high strength ceramic materials with enhanced fracture toughness may facilitate the development of minimally invasive preparation technique. (4)

The shape and thickness of the core/framework of a restoration has been proposed to be of importance for fracture behavior. With the development of computer aided design/computer aided manufacturing (CAD/CAM) production, blocks of Y-TZP materials can be milled into crowns and FDP frameworks with good precision and clinically acceptable fit, within reasonable time and at an acceptable cost (5)

Early CAD/CAM created restorations unfortunately was with a simple design and with an even thickness of material. Limitations in the software meant that the final shape of the restoration, support for the veneering material, and antagonist shape were not given full consideration. Modern CAD/CAM systems have few limitations, and in-vitro studies have suggested that an adapted design of the substructure results in higher load at fracture and fewer and smaller veneer fractures (6, 7) .

resistance was obtained with (Upward connector with 0.3 mm finish line and 0.5 mm coping) 1706.9N, (Straight connector with 0.5 mm finish line and 0.5 mm coping) 1647.6 N, then (Straight connector with 0.3 mm finish line and 0.5 mm coping) 1513.2 N with a statistically significant difference between the three groups. Significantly lower mean fracture resistance was found with (Upward connector with 0.3 mm finish line and 0.3 mm coping) 997N. The statistically significantly lowest mean fracture resistance was found with (Straight connector with 0.3 mm finish line and 0.3 mm coping) 985.1N.

Conclusion: The fracture resistance of zirconia fixed partial dentures is dependent on finish line thickness, coping thickness, as well as connector design.

FRACTURE RESISTANCE OF DIFFERENT YTTRIUM ZIRCONIA POSTERIOR (1759)

The mechanical properties of zirconia are the highest ever reported for any dental ceramic. This may allow the realization of posterior fixed partial dentures and permit a substantial reduction in core thickness. (8, 9)

Fracture of all ceramic FPDs tends to occur in the connector area because of stress concentrations. The shape of the FPD is not uniform. Its contour has a complex combination of multiple convexities and concavities, depending on the geometry of the teeth and their alignment. Unfortunately, the connector areas of FPDs are generally limited in the posterior regions, due to biologic reasons. These sites in the FPDs represent stress concentrations relative to the average stress levels within other areas of the prosthesis. (10)

Many studies found that survival rate of all ceramic bridges is totally affected by the design and dimensions of the connector area. (11,12) If the connector design can be altered in regions where maximum tension occurs, the characteristics stress pattern can be optimized to improve the survival time of all ceramic fixed partial dentures. Therefore; great concern should be given to the connector area in all ceramic bridges especially in posterior region.(13-15)

The effects of different axial coping and finish line thicknesses , as well as connector designs on the fracture resistance of yettrium zirconia posterior fixed partial dentures framework was evaluated in this study.

MATERIALS AND METHODS

Materials used in this investigation

1) Sirona in coris ZI 55/19 (Zirconia) blocks. (Cerec Inlab, sirona dental system Bensheim)

2) E-max Zirpress ingots: (Ivoclar Vivadent Schaan Liechtenstein)

3) Self adhesive resin cement (Rely X unicem) (3M™ ESPE™)

Methods

CAD/CAM Cerec in-Lab system (Cerec stone BC, Sirona Dental Services GmbH, Germany) was used to mill thirty bridge frameworks which are divided into three groups (each of 10 frameworks) according to finish line and coping thicknesses:

Group 1: (n=10)

Chamfer finish line thickness 0.3mm, coping thickness 0.3mm.

Group 2: (n=10)


Group 3: (n=10)


Each group was further subdivided into 2 equal subgroups(n=5) according to the design of the connector area:

Subgroup A: (5 samples) The connector is directed occlusaly.

Subgroup B: (5 samples) The connector is straight.

Models construction:

2 stainless steel models were fabricated. Each model resembled a prepared lower second premolar and lower second molar with an edentulous span area for missing lower first molar. The first model is milled with 0.3mm chamfer finish lines for both abutment, while the second model is milled with 0.5 mm chamfer finish line for both abutment. (Fig 1)

Both models are duplicated using polyvinyl addition silicon material (3M™ ESPE™) utilizing double mix one step technique. Then the impressions were poured with special cerec stone (Cerec stone BC, Sirona Dental Services GmbH, Germany) recommended by the manufacturer . The stone were mixed following the manufacturer instructions then


the impression was poured under vibration. After stone setting the models were separated from the impression. The excess stone was trimmed, and the models were now ready for scanning. The stone models were further duplicated with polyvinyl addition silicon material utilizing double mix one step and poured with epoxy resin (Chemapoxy 150, CMB) mixed according to manufacturer instruction and left to set for 24 hours before separation from the impression. The epoxy resin model were separated from the impression and trimmed.

Model scanning and framework designing:

The models were scanned by the Cerec in-Lab system, then frameworks designs were adjusted regarding the axial wall thickness and connector thickness. The coping thickness was adjusted by clicking on the icon of coping thickness to be 0.3 mm for the first group, and 0.5 mm for the second and third group. For the second group the collar thickness is adjusted to be 0.3mm while the coping is then adjusted to be 0.5 mm.

The connector was clicked and the thickness was adjusted to be 9mm2, then the connector design was adjusted by clicking the rotate icon in order to direct the connector upward. (Fig 2), (Fig 3)

Milling and sintering of the frameworks:

To start the milling procedures, the type of the block was selected from the Select block box

by clicking In Coris ZI 55/19. The block type was selected and confirmed by OK. The selected ceramic block was inserted in the spindle of the milling chamber of the Cerec In-Lab system and fastened with the set screw. This was followed by the closure of the chamber door. The Mill icon was then clicked. The restorations were milled with an oversize of approximately 20-25% and shrunk subsequently to the exact fitting final contour in the sintering process. The exact shrinkage data of the respective block was stored in a barcode on the block itself, which was automatically read prior to the milling process.

The sintering firing is authorized only in the high-temperature furnace Vita Zyrcomat (Vita Zahnfabrik, DZY220, Bad Sackingen, Germany).

Fig. (1): Stainless steel models Fig. (2) Connector directed occlusaly

Fig. (3) Straight connector


The frameworks were placed into the sintering bowl Zyrcomat (Vita Zahnfabrik, DZY220, Bad Sackingen, Germany) which was then placed in the center of the firing tray. The furnace was closed and the Start key was pressed. The duration of the sintering program was approximately 7.5 hours including the cooling phase to 200˚C and the maximum temperature reached was 1500˚C. After sintering the frameworks were placed on their corresponding models to ensure optimal adaptation.

Veneering of the frameworks

• For standardization the frameworks were veneered using press on zirconia technique. Wax patterns for the veneering was designed utilizing CAD/CAM software technology. The thickness of the veneering was adjusted to be 1mm thickness and designed with proper occlusal anatomy. Then the veneering was then milled from prefabricated wax blocks. The wax patterns were then checked for their passive fit on their corresponding frameworks. ( Fig 5)

• IPS e.max Zirliner (Ivoclar Vivadent Liechtenstein) were applied on the entire frameworks in order to mask the opacity of the zirconia frameworks and to insure optimal bond with the pressed veneering. After that, the applied ZirLiner was dried, fired on a honey-

combed firing tray at 960 c. After firing, the IPS e.max Ceram ZirLiner should exhibit a layer thickness of approximately 0.1 mm. Wax sprues of diameter 3 mm and length 4-6 mm were attached to the thickest portion of wax patterns using sticky wax. Taking into consideration that the distance between the cervical part of the wax patterns and the top surface of the silicon ring was 10 mm.The wax sprue formers were attached to the crucible former using sticky wax. The silicon ring is fixed to the crucible former, and special phosphate bonded investment were mixed (IPS PressVEST Speed) (Ivoclar Vivadent Liechtenstein) with proper P/L following the manufacturer instruction using vacuum mixer (350rpm) for 2.5 minutes. The investment was applied on the wax pattern and inside the fitting surface of the zirconia frameworks utilizing a small brush, then the rest of investment was poured inside the ring under vibration without entrapment of air bubbles. The investment was left to set for 45 minutes, the ring and the crucible former were removed gently, and the investment block was introduced inside the burn out furnace (Ivoclar Vivadent Liechtenstein) for wax elimination at 850 c. After wax elimination, IPS e.max zirpress (Ivoclar Vivadent Liechtenstein) ingots were introduced inside the hot investment ring followed by the Alox plunger (Ivoclar Vivadent

Fig. (4) Zirconia framework seated on their correspanding models.

Fig. (5) Milled wax pattern for the veneering


Liechtenstein) and the ring was then inserted in the center of the press furnace (Ivoclar Vivadent Liechtenstein).

The pressing cycle was started, pressing was done at 910°c . After pressing cycle was finished, the ring was removed from the pressing furnace and the investment was cut using separating disc, and the rest of investment was removed using polishing beads.. The sprues were cut off using fine diamond disc the reaction layer formed during the press procedure was removed using IPS e.max Press Invex Liquid (Ivoclar Vivadent Liechtenstein). The veneer was finished using finishing cups cup, stained and glazed at 770 °c (Fig 6)

Cementation of the bridges

Sandblasting of the surfaces to be cemented was performed using 50μm Al2O3 at a sandblast-ing pressure of 2.5 bars for 10 seconds. The resto-rations were ultrasonically cleaned in 96% alcohol and then rinsed with water for 30 seconds and dried. All restorations were cemented onto their corre-sponding epoxy resin models using Rely-X Uni-cem (3M™ ESPE™), a dual-polymerizing resin cement, according to the manufacturer instructions. Rely-X Unicem tube was clicked to extrude an equal amount of base and catalyst then mixed with

a plastic spatula on a clean mixing pad. An even thin layer of the resin cement was applied to the internal aspect of the retainers. Each bridge was seated on its corresponding epoxy resin model using steady finger pressure, making sure they were completely seated. Excess cement along the margins was gently removed with soft brush. The seated bridge with its epoxy resin model was then transferred immediately to the cementing device. A resin occlusal template, 5 mm in thickness was particularly tailored for each ceramic bridge, so that the applied static pressure (5Kg) would be equally distributed on the bridge units during the curing procedures. All the margins were light polymerized for 20 seconds at 5-mm dis-tance and an irradiation intensity of 130 mW/cm2 for each surface of every retainer. One hour after cementation, all specimens were stored in distilled water bath for one week before testing.

Fracture resistance testing

All samples were individually & vertically mounted in the lower fixed compartment of a computer controlled materials testing machine (Model LRX-plus; Lloyd Instruments Ltd., Fareham, UK) with a loadcell of 5 kN and data were recorded using computer software (Nexygen-MT; Lloyd Instruments). Force was applied with a custom made load applicator [steel rod with spherical tip (5.8 mm) placed at the center of the occlusal surface of pontic] attached to the upper movable compartment of the machine. A tin foil sheet was placed between the load applicator and the specimen to ensure even stress distribution and minimization of the transmission of local force peaks. The samples were subjected to a slowly increasing vertical load (1mm/min) until the fracture. (Fig 7)

Failure was manifested by an audible crack sound and confirmed by sudden drop along the load-deflection curves recorded using a computer software.

Fig. (6) Press on zirconia veneer


Statistical analysis

Data were presented as mean and standard deviation (SD) values. Regression analysis using two-way Analysis of Variance (ANOVA) was used for studying the effect of design, material of construction and their interactions on mean fracture resistance. Tukey’s post-hoc test was used for pair-wise comparison between the mean values when ANOVA test is significant.

Detailed comparison between the different designs was performed using one-way ANOVA followed by Tukey’s test. Detailed comparison between materials of construction was performed using Student’s t-test.

The significance level was set at P ≤ 0.05. Statistical analysis was performed with IBM (IBM Corporation, NY, USA) SPSS Statistics Version 20 for Windows (SPSS, Inc., an IBM Company).

RESULTS

The statistically significantly highest mean fracture resistance was found with (Upward connector with 0.5 mm finish line and 0.5 mm coping) 2307.3N. Lower mean fracture resistance was obtained with (Upward connector with 0.3 mm finish line and 0.5 mm coping) 1706.9N, (Straight connector with 0.5 mm finish line and 0.5 mm

coping) 1647.6 N then (Straight connector with 0.3 mm finish line and 0.5 mm coping) 1513.2 N with a statistically significant difference between the three groups.

Significantly lower mean fracture resistance was found with (Upward connector with 0.3 mm finish line and 0.3 mm coping) 997N. The statistically significantly lowest mean fracture resistance was found with (Straight connector with 0.3 mm finish line and 0.3 mm coping) 985.1N.

TABLE (1): Mean, standard deviation (SD) values and results of one-way ANOVA and Tukey’s tests for comparison between fracture resistances of all groups

ConnectorFinish line thickness

Coping thickness

Mean SD P-value

Straight0.3 mm

0.3 mm 985.1 f 4.8

<0.001*

0.5 mm 1513.2 d 8.2

0.5 mm 0.5 mm 1647.6 c 3.1

Upward0.3 mm

0.3 mm 997 e 2.10.5 mm 1706.9 b 1.4

0.5 mm 0.5 mm 2307.3 a 9.6

*: Significant at P ≤ 0.05, Different letters are statistically significantly different

Fig. (7) Fracture resistance testing

Diagram (1): Bar chart representing mean values for comparison between fracture resistance of all groups


DISCUSSION

Currently, dental prosthetic treatments follow principles based on preserving sound tissue, generally requiring the removal of limited amounts of sound tooth structure, including axial reduction and the finish lines. To provide a restoration with the necessary retention and resistance, the axial walls must be slightly tapered and cement must be placed in compression. Modern adhesive technologies and high strength ceramic materials with enhanced fracture toughness may facilitate the development of minimally invasive preparation techniques. (16)

Yttrium tetragonal zirconia polycrystal (Y-TZP) –based systems are a recent addition to the high-strength all ceramic systems used for crowns and fixed partial dentures in stress bearing region. The esthetic nature of zirconia coupled with its superior physical properties and biocompatibility have resulted in restorative systems that meet the demands of today¢¢s patient. Undoubtedly, these systems are considered to be prospective replacements for metal-ceramic restorations.(17)

Until now, clinical results have shown that all-ceramic restorations fabricated by using zirconia coping showed high success rate after long-term function, only minor chipping was detected particularly for molar region. There may be some reasons to fail of veneered porcelain; flexural strength, bond strength between coping and porcelain, excessive load, porosities and surface conditions of the porcelain, improper coping design, and thermal stress when firing porcelain.(9,18)

Also, there might be some factors that will influence the fracture incidence of all-ceramic restorations. Many studies revealed that the fracture load will depend on coping thickness, marginal design, and applied luting agent as well as connector dimension. (19,13,18)

In this research the hypothesis that the effects of different axial coping and finish line thicknesses, as

well as connector designs on the fracture resistance of yttrium zirconia posterior fixed partial dentures frameworks were evaluated.

In this study 2 models were milled representing prepared lower second molar and prepared lower second premolar. The bucco-lingual as well as mesio-distal dimension of the prepared abutment was similar to actual natural prepared abutment in the clinical situation. (20)

Moreover; the occluso-gingival height, degree of convergence and the length of edentulous span were adjusted according to previous studies (21,22, 23,24, 25).

Cerec in-lab CAD/CAM system was used to mill the zirconia frameworks. The clinical performance and longevity of all ceramic restorations fabricated by cerec-inlab show ultimate success results. (26),(27)

Cerec in-lab offers a complete spectrum of design option for crown and bridge frameworks. All the relevant parameters can be adjusted individually, for example, the occlusal and axial wall thicknesses and the cross sectional area and design of connectors. The operator thus exerts complete control over the design process all the time.

Y-TZP restorations can be made from blocks of fully-sintered Y-TZP or partially-sintered Y-TZP. Manufacturers claim that fully sintered Y-TZP is stronger and tougher than partially sintered Y-TZP, and in-vitro studies comparing fully sintered and partially sintered Y-TZP materials have found significantly higher fracture resistance for the fully sintered material (28).

Although of its higher fracture resistance, shaping the dental restoration from a fully sintered material, demands more time and powerful cutting instruments and there might be a risk of introducing surface flaws during this procedure. Studies have found that grinding of a Y-TZP ceramic induces surface flaws and microcracks and may significantly decrease strength. (29, 30)


The type of zirconia used in the research was partially sintered one. Producing a restoration in partially sintered Y-TZP is easier as most cutting is done in this softer stage which is less time-consuming and does not demand the same powerful cutting. The final framework is thereafter fully-sintered and potential flaws created in the surface during processing may then be closed which could be a possible advantage for partially sintered Y-TZP materials. Studies have found however, that surface and subsurface micro-cracks are not fully healed by the final sintering process (31) .

The final sintering involves a shrinkage that needs to be compensated, and there has been some concern as to whether this affects marginal fit. One study on a partially sintered Y-TZP material has reported a high incidence of secondary caries (32) , but this material was a prototype and the production methods have since been improved and the marginal fit of Partially sintered Y-TZP restorations has been found clinically acceptable in other studies (33) .

All the procedures of impression making and the different construction techniques till obtaining the all-ceramic bridges were carried out following the manufacturer instructions. Proper seating of the bridges on their respective stone master models and epoxy-resin models was verified using a magnifying lens.

Airborne particle abrasion with 50 µ Al2O3 abrasive particles was performed on the fitting surface of all ceramic retainers before cementation as it had been proven to be an effective method for micro-mechanical bonding of the adhesive resin cements with zirconium-oxide ceramics. In addition, it was proven that airborne particle abrasion increased the flexural strength of the Y-TZP ceramics as it was able to generate a tetragonal to monoclinic phase transformation of the grains on the surface of Y-TZP ceramic and to create a layer of compressive stresses counteracting the effective depth of the surface flaws induced by airborne particle abrasion(34)

The zirconia bridges were cemented to the epoxy resin dies using Dual cured resin cement (relyX unicem). Although many authors recommended the use conventional cement for cementation of zirconia restorations as improved resin adhesion has been reported when using tribochemical silica coating and silanization of the zirconia frameworks, or when using phosphate-modified resin luting agents.(35,36,16) However the self adhesive cement was used to simulate clinical situation when chemical bond to tooth structure was required in order to achieve an ultimate margin fit and clinical longevity. (34)

To ensure even, homogenous cement film with optimal thickness between the zirconia bridges and the epoxy resin models a special loading device was constructed with 5 kgs weight and used to gaurentee standardization of pressure applied on the samples during cementation providing optimal cement space for resin luting cements.

All the samples were fractured using computer controlled materials testing machine (Model LRX-plus; Lloyd Instruments Ltd., Fareham, UK). A double layer of tin foil was placed between the loading stamp and the pontic to achieve homogenous stress distribution and minimize the transmission of local force peaks from the loading stamp to the occlusal surface, as reported by Ohlmann et al (2005) (37), Att et al (2006) (38) and Wolfart et al (2007) (39).

The use of ceramic cores to replace metal frameworks has been of great concern to dentists for long time. The clinical fracture resistance of all-ceramic FPDs is related to the size, shape and position of the connectors and the span of the pontic. When occlusal forces are applied directly to the pontic through the long axis of an all-ceramic fixed partial dentures, compressive stress occurs at the occlusal aspect of the connector and flexural stresses concentrates at the gingival embrasure of the connector. (40)


In-vitro tests and finite element analyses have shown that the greatest stress in an all-ceramic FDP occurs in the connector area (11,12). This is true also for Y-TZP-based FDPs. Although complete fractures are rare in clinical evaluations of Y-TZP-based restorations (41), However, connector design and dimensions needs to be considered. Regarding connector dimension in this study was 9mm2 as recommended by many authors (13,14,15)

The upward connector showed statistically significant higher fracture resistance values compared to the straight connector for both finish lines and coping thicknesses. Tsumita, et al.(42) stated that the concave shape caused the highest maximum tensile stress, and high tensile stresses were appeared at the lower surface of pontic. Researchers also reported that convex design reduced the stress concentration at the connector area of the gingival embrasure. Kokubo, et al.(40)

evaluated the effect of straight, convex and concave pontic designs on the fracture resistance of implant-supported all-ceramic fixed partial dentures and declared that convex design is particularly useful for molar region. The results of present study agree with those of Tsumita, et al,(42) and Kokubo, et al.(40) It is known that porcelain is weaker when stressed at tension and it is much stronger under compression.(39) Tensile stresses tend to occur at lower surface of concave or straight beams by loading. (42) Also, the convex arches are the most efficient method of forming a structure with materials that have good compressive strength and low tensile strength. (42) Therefore, highest values of fracture resistance with respect to the other connector designs studied were observed at convex design, while straight design showed lowest values of fracture resistance.

It has been reported that the mean adult occlusal force is about 400 to 800 N at the molar region, 300 N at the premolar region, and 200 N in the

anterior region (43) . Oh and Anusavice (44) stated that clinical contact areas on the pontic and the adjacent abutments may generate variations in the mode of failure in all-ceramic 3-unit bridges and ceramic prostheses may fail at values far lower than the mean values measured intraorally. All loads in this study were applied axially at the center of the pontic. The mean fracture loads for both designs were found higher than the mean maximum masticatory forces for molar region.

The ideal finish line design for all ceramic restoration is deep chamfer with thickness of 0.7-1mm. However many trials were made to decrease the thickness of the finish line as well as the coping thickness in order to conserve the tooth structure especially with the development of high strength all ceramic restorations.(45,46)

In this study decreasing the coping thickness to 0.3 mm with 0.3mm finish line show significant decrease in fracture resistance ( 997.1N) even with the connector design directed upward. This was in agreement with the manufacturer instruction which recommend the miniumim thickness thickness of zirconia framework to be 0.5 mm. however, this was on the contrary with Sven et al (16) who claimed that knife edge preparation present a promising alternative to chamfer finish lines, and the fracture load required for knife edge preparation was 38% greater than that required for chamfer preparation regardless coping thickness .

However increasing the coping thickness to 0.5mm with 0.3mm finish line show statistically significant higher fracture resistance compared to the 0.3 mm coping thickness even with the straight connector design (1513.2N).

Coping thickness of 0.5mm with 0.5 mm finish line chamfer design show statistically significant higher fracture resistance for both connector design straight and upward (1647.6N and 2307.3N respectively). This was in agreement with Narong et al and Yuji et al. (45,18)


CONCLUSIONS

From the results of the present in vitro investigation and within its limits the following conclusions can be drawn:

1- The fracture resistance of zirconia fixed partial dentures is dependent on finish line thickness, coping thickness, as well as connector design.

2- Connector design directed upward show higher fracture resistance load values regardless finish line and coping thickness.

3- Finish line and coping thickness of 0.5 mm show higher fracture resistance load value compared to finish line and coping thickness of 0.3mm.

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FRACTURE RESISTANCE OF DIFFERENT YTTRIUM ZIRCONIA ...scholar.cu.edu.eg/?q=ahmedidris/files/dr.ahmed_idris.pdf · the impression was poured under vibration. After stone setting the