SINTERING EFFECT ON THE MARGINAL FIT AND FLEXURAL STRENGTH OF ZIRCONIA CROWNS: SYSTEMATIC REVIEW OF LITERATURE

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Balsem Ahmed Sabah

2020, Group 12

SINTERING EFFECT ON THE MARGINAL FIT AND

FLEXURAL STRENGTH OF ZIRCONIA CROWNS:

SYSTEMATIC REVIEW OF LITERATURE

Master’s thesis

Supervisor Assistant, Rimantas Ožiūnas

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LITHUANIAN UNIVERSITY OF HEALTH SCIENCES MEDICAL ACADEMY

FACULTY OF ODONTOLOGY PROSTODONTICS CLINIC

Sintering Effect on the Marginal Fit and Flexural Strength of Zirconia Crowns: Systematic Review of Literature

Master’s thesis

The thesis was done by

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ABSTRACT

Aim: The study aim is to evaluate different articles that assess the effect of change in sintering

parameters on the properties of zirconium material.

Materials and methods. Electric search was conducted using the Medline database, Science direct,

EBSCO publishing, and Wiley Online Library for finding studies concerning sintering effect on marginal fit and flexural strength of zirconium. A total of 535 articles were found. To narrow the specific search keywords were used, and the search was restricted to the English language, dated from 2010 to 2020, and followed the inclusion and exclusion criteria. The articles were then reviewed, and duplicates were removed. In the end, nine articles were left and included in this study.

Results. For this systematic review, a total of eight articles were analyzed. Two articles revealed no

significant clinical effect of change in sintering parameters on the marginal accuracy of zirconia specimen. A total of 4 articles showed that an increase in sintering temperature from 1400 °C and above caused flexural strength to increase, however after a certain temperature of about 1600°C, a decrease in strength was noted. The last three articles showed no significant difference in flexural strength upon changing sintering parameters.

Conclusion. 1. From the two articles included, we can conclude that there was no significant

difference in marginal adaptation when changing sintering parameters. Application of shorter sintering time is acceptable since the marginal discrepancy values were within the clinically acceptable range in the included studies.

2. Sintering temperatures between 1550°C and 1580°C caused an increase in flexural strength. However, temperatures above 1600°C caused a decrease in flexural strength.

3. Shorter sintering times did not cause a change in the flexural strength of zirconia material, and can, therefore, be utilized in dental practices.

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INTRODUCTION

Nowadays, several people carry a form of medical implant, such as dental implants. The crowns that these implants support are essential for satisfying patient’s needs both esthetically and functionally. Zirconium has shown to be a promising restoration and an alternative as a non-metal restoration; furthermore it is a biocompatible material that has both promising mechanical and esthetical properties [1]. These restorations have become the better alternative and the more preferred choice of material. Lower amount of tooth tissue removal of abutments accompanied by only a small amount of wear on opposing teeth are the main advantages of zirconium restorations when compared to other all-ceramic restorations [2, 3].

Zirconia consists of polycrystalline particles in a metastable tetragonal crystal structure. It is considered to have the highest fracture strength of all dental ceramic materials that we have

today [4, 5]. It can exist in three forms: the monoclinic form that is stable at a temperature of

1170°C, the tetragonal phase is stable at 1170°C - 2370°C. Beyond 2370°C zirconia assumes its

cubic form and reaches the melting point at 2680°C [6, 7, 8]. It is a material used in various dental

procedures such as all-ceramic copings, fixed partial and complete prosthesis [9].

The three generations of zirconia material are sintered at temperatures starting from lower to higher

temperatures. First generation is tetragonal zirconia polycrystals, 3Y-TZP, with small grain size and

high flexural strength. Partially stabilized zirconia, 3Y-PSZ, is considered second generation and

have bigger grain size with decreased strength. Last generation is 4-5Y-PSZ, it has a grain size bigger than second generation and is more brittle and weak when compared to the others. High flexural strength materials give restorations a lower risk of fracture. Most of our dental restorations are subjected to masticatory loads and flexural strength signifies the stresses required to either fracture or plastically deform a restoration. Therefore an excellent flexural strength is beneficial to withstand such forces. The flexural strength is mainly dependent on the grain size of zirconia [9, 10]. Alteration in the sintering parameters can influence the strength of these restorations [10].

Zirconia prostheses are usually processed by Computer-aided design and computer-aided manufacturing. Most commonly used are the partially sintered zirconia blanks that are easier to mill in computer-assisted manufacturing. Afterward, it must be further sintered for the highest density, strength, and reaching the fully sintered form [9, 12]. This procedure causes about 20 % to 30 %

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3 volumetric shrinkage [9]. To be able to compensate for this sintering shrinkage, the restoration needs to be in enlarged dimension before milling [12].

On the other hand, zirconia can also be milled using fully sintered zirconia blanks. Usage and milling of a fully sintered blank cause more accuracy to the restoration, this is due to not requiring any further sintering process, and there is no shrinkage of zirconia. One of the factors that play a central role in the success of a restoration is its marginal fit [12]. Post machining heating can cause some form of shrinkage, and in this way, the sintering process can possibly affect the marginal accuracy of a restoration [12, 10].

Usually, three phases make up the sintering process; it has a heating phase, sintering phase, and lastly a cooling phase [11]. Sintering of zirconia is usually a time-consuming procedure that involves both slow heating and cooling rates followed by an extended dwell time, usually 2-5 hours. The typical sintering procedure has a final sintering temperature between 1350 °C - 1550°C [13]. It is through the alteration in the sintering process that we can optimize the properties of zirconia. .The mechanical properties of zirconia ceramics can be impacted by changes in parameters, specifically the changes in the sintering time and temperature [11]. A few studies evaluated the sintering time and temperature effect on zirconia marginal accuracy and flexural strength.

Aim: The aim of the study is to evaluate multiple articles which examine the effect on the properties of zirconium material when changing time and temperature of sintering procedures.

Tasks:

1. To find out how the increase and decrease in sintering temperature and time can affect marginal fit.

2. To assess how increase and decrease in sintering temperature can affect the flexural strength.

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1. SELECTION CRITERIA OF STUDIES. METHODS AND

STRATEGY

1.1 Method

Bioethics approval code - BEC-OF(U)-118.

The search was conducted in the following databases : Pubmed, Sciencedirect ,EBSCO publishing and Wiley Online Libray. The search was conducted using a combination of keywords such as ‘‘Sintering temperature‘‘,‘‘Sintering effect‘‘,‘‘Flexural strength‘‘ ,‘‘Zirconia‘‘ and ‘‘marginal fit‘‘. Search was narrowed down further by the usage of the exclusion and inclusion criteria. During article search in Pubmed database , a set of different keywords were used ,thus instead of using‘‘sintering effect‘‘ like with other databases,the keyword ‘‘sintering temperature‘‘ was applied. Furthermore the combination of flexural strength and marginal fit was not applied during article search on Pubmed. This slight change yielded better results than the keywords used in the other databases.This is a review of in vitro studies which evaluated the effect of change in sintering parameters on marginal accuracy and flexural strength of zirconia material.

1.2 Selection criteria of studies

Table1. Inclusion , exclusion criteria

Study type

Inclusion criteria Exclusion criteria

In vitro studies

Original articles full text English language

Studies on animals, humans Systematic reviews

Litterature review Summaries Abstracts

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1.3 Online Search in databases

Table 2. Pubmed database

Date:14.11.2019

Keywords Filter Results

sintering temperature flexural strength zirconia

No 31

10 years 26

English language 24

Full text 24

Table 3. Pubmed database

Date:14.11.2019

sintering effect on marginal fit zirconia

No 7

sintering effect on marginal fit

zirconia

10 years 5

sintering effect on marginal fit

zirconia

English 4

sintering effect on marginal fit zirconia

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6

Table 4. ScienceDirect

Date:14.11.2019

sintering temperature flexural strength or marginal fit zirconia

No 179

10 years 130

English (not applicable)

-

Full text (not applicable)

-

Table 5. EBSCO Publishing

Date:14.11.2019

No 161

10 years 137

English 128

Full text 46

Table 6. Wiley Online Library

Date:14.11.2019

No 157

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7 strength or marginal fit zirconia

English (not applicable)

-

Full text (not applicable)

-

1.4 Study selection

The search strategy was conducted through electronic databases and a combination of keywords were used for the search. To narrow the search, the following keywords were used:

1)Sintering effect 2) Sintering temperature 3) Zirconia 4) Marginal fit

1.5 Risk of bias

A limatition of this study is that it inlcudes data from in vitro studies , these studies are carried out using machines and tests which cannot be a cause of having higher risk of bias. There was no good published risk of bias approach or establsihed framework for confidence rating for this systematic review.

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2. SYSTEMIZATION AND ANALYSIS OF DATA

2.1 Results

Author, year

Title Time Temper

ature (°C) Results Conclusion Khaledi Amir et al. [11] 2017

“The Effect of Sintering Time on the Marginal Fit of ZirconiaCopings” 75 minutes 4 h 20 min 7 h 20 min 1530°C 1530° C 1530°C Mean values (μm) 41.06 ± 14.03 43.03 ± 11.67 39.88 ± 15.23 There was no significant change in marginal fit. Walaa Magdy Ahmed et al. [14] 2019 “Marginal Discrepancies of Monolithic Zirconia Crowns: The Influence of Preparation Designs and Sintering Techniques“ Groups1-6: Standard sintering 9h , 50min Groups 7 - 12 : fast sintering 2h, 55min 1350 C 1600 C

Group 8 (Fast sintering): 47.9 (1.66) μm

Largest vertical marginal gap mean.

G3 and G4(standard sintering):

14.43 (1.66) μm and 14.65 (1.66) μm, Smallest vertical marginal gap mean. No significant vertical marginal gaps changes. However highest marginal gaps between groups the groups of fast sintering Nuri Murat Ersoy et al. [15] 2015

“The effects of sintering temperature and duration on the flexural strength and grain size of zirconia“ Group A/D :8h, Group B/E: 2h Group C/F: 10 min 1510 °C 1540 °C 1580 °C Mean (MPa) Group A: 700.3 Group B :662.1 Group C: 871.8 Group D: 579.7 Group E: 622.3 Group F: 904.2 Highest flexural strength was observed in zirconia sintered at 1580°C within 10 min. Bogna Stawarcz yk et al. [16] 2013

“The effect of zirconia sintering temperature on flexural strength, grain size and contrast ratio “

2h 1,300°C 1,350°C 1,400°C 1,450°C 1,500°C 1,550°C 1,600°C 1,650°C 1,700°C

Mean (standard deviation values,SD) 969.8 (157 Mpa) 950.9 (201 MPa) 1119.3 (143 MPa) 1214.5 (194 MPa) 1281.1 (230MPa) 1256.7 (165MPa) 979.2 (218MPa) 856.4 (168MPa) 585.6 (251MPa) Highest flexural strength at sintering temperatures between 1400°C and 1550°C.

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10 Niwut Juntavee et al. [10] 2018 “Effect of different sintering process on flexural strength of translucency monolithic zirconia“ 60 min (1h, HS) 120 min (2h, HR) 180 min (3h, HP) 1350°C(SD, decreased) 1450 °C (SR, regular) 1550 °C (SI,increase d) Mean (SD) SDHS=1080.25(217.19) SDHR=1243.41(233.17) SDHP=1298.92(235.68) SRHS=1303.34(171.87) SRHR=1331.73(278.84) SRHP=1348.13(283.35) SIHS =1458.45(289.19) SIHR =1581.34(190.56) SIHP =1604.10(139.52) Higher sintering temperatures and longer sintering time gave higher flexural strength. Jan Ulrich Jansen et al. [17] 2019 “Impact of high-speed sintering on translucency, phase content, grain sizes, and flexural strength of 3Y-TZP and 4Y-3Y-TZP zirconia materials“ 1h to 2h (controll sintering) 10 minutes (high-speed sintering) 1450 °C 1570 °C 1590 °C

Material Temp (MPa) ZI- 1570 1251 1590 1241 1450 1251 ZD- 1570 1023 1590 1220 1450 1080 HT+ 1570 1146 1590 1257 1450 1126 Higher flexural strength was reached at high speed sintering of 1590°C. Kátia Vieira Cardoso et al. [18] 2019 “Effect of sintering temperature on microstructure, flexural strength, and optical properties of a fully stabilized monolithic zirconia“ 2h 1450 °C 1600 °C Mean (MPa) 542.9 ± 112.2 577.5 ± 99.3 No evident effect on flexural strength was seen by change in sintering temperature. Kamal Ebeidet al. [19] 2014 “Effect of changes in sintering parameters on monolithic translucent zirconia“ 1 h 2 h 4 h 1460 °C 1530 °C 1600 °C

Flexural strength ranged between: 906 MPa to 1000 MPa for all temperatures and times.

There was no effect on flexural strength when changing sintering parameters. Caner Özturk et al. [20] 2019 “Effect of sintering parameters on the mechanical properties of monolithic zirconia“ 30 min 60 min 120 min 240 min Controll 1400°C 1450°C 1500°C 1600°C

Group TZI Group Up 488.50 586.54 354.68 370.23 407.94 625.96 472.48 427.30 345.48 415.18 379.96 489.61 478.05 464.12 414.21 521.62 457.53 433.50 397.60 406.28 439.63 510.60 436.01 550.28 435.35 439.15 522.95 490.22 417.98 449.28 542.80 545.71 551.77 529.04 No change in in the flexural strength of zirconia.

Table 6. Characteristics of included studies with results: effect of different sintering times and temperatures on marginal fit and flexural strength.

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2.2 Study characteristic

Khaledi Amir et al. [11], the authors studied the effect of various sintering times on the marginal adaptation of zirconia copings. The copings were milled and sintered in a furnace at three different times. The ANOVA test was used to calculate the marginal gaps for the three groups, and the result showed no major discrepancies in the marginal gap in all three groups.

Wala Magdy Ahmed et al. [14] also assessed the sintering effect on multiple abutments and their marginal fit, using alternation between the standard sintering technique and the fast sintering technique. The abutments were split into twelve groups, and the crowns in groups 1-6 were sintered using conventional sintering. Fast sintering was utilized and used on groups 7-12. Study showed that the vertical marginal gaps were in the clinically acceptable range; however the groups of fast sintering demonstrated the highest vertical marginal gaps as to the standard sintering groups.

Nuri Murat Ersoy et al. [15] and his fellow authors examined how changes in sintering parameters cause changes in flexural strength. The used zirconia specimen were partially sintered and further divided into three random groups according to their sintering time and temperature. The flexural strength was then measured using the ISO 6872 2008 standards and a formula. The results showed that the usage of elevated sintering temperatures, as well as short sintering time, led to an elevation in the flexural strength. Zirconia sintered at 1580°C within 10 min had the highest flexural strength. Bogna Stawarczyk et al. [16] evaluated the change in sintering temperature on the flexural strength. The zirconias were in a partially sintered state and divided into nine groups according to different sintering temperatures. It was observed that group I (1700°C) had the lowest flexural strength, and the highest flexural strength was seen at sintering temperatures of 1400 °C to 1550°C.

Niwut Juntavee et al. [10], like Nuri Murat Ersoy et al. [15], investigated the impact of both sintering time and temperature on the flexural strength. However, Niwut Juntavee et al. [10] used a higher number of specimens than Nuri Murat Ersoy et al.; a total of 135 partially sintered zirconia specimens were used and randomly divided into six groups. The result was similar to those of the previously mentioned article, thus altering the temperature and time affects the flexural strength. The highest flexural strength proved to be in the group SIHP, which stands for increased sintering temperature of 1550°C and a duration of three hours.

Jan Ulrich Jansen et al. [17], in this article, the effect of change in sintering time and temperature on flexural strength was evaluated. However, unlike the other studies, it also showed how different zirconia materials were affected. The materials chosen were 3% mol Y-TZP and 4% mol Y-TZP. When it comes to sintering at 1590 °C, the study showed that there was no difference in the choice of material and strength during the fast sintering protocol. Zirconia specimen sintered at 1570 °C

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12 and 1450 °C showed a difference, where ZI showed higher biaxial flexural strength results than ZD material. The ZD material showed higher flexural strength at 1590°C than at 1570 °C

Kátia Vieira Cardoso et al. [18] used pre-sintered, fully stabilized zirconia blocks that were further divided into two groups and sintered at 1450°C and 1600°C. However, in contrast to the results of the other studies, this study showed no significant change in the flexural strength when altering the sintering temperature. Similarly, Kamal Ebeid et al. [19] found no significant changes in flexural strength when using the selected three temperatures and three holding times.

3. DISCUSSION OF RESULTS

Nowadays, zirconium has become the preferred choice of restorative materials. It is a promising restoration, is biocompatible, with excellent mechanical properties, and most importantly satisfies patients’ desires for natural appearance. The present review evaluated the effect of change in sintering parameters on zirconium particularly on the changes in marginal accuracy and flexural strength of zirconia copings. A total of 9 articles were selected and analyzed with results presented in Table 6.

Alteration in sintering time and temperature can help milled zirconia blocks to improve in strength; furthermore, altering time can help achieve faster manufacturing of zirconia prosthesis [15]. For the creation of the final structure of zirconia prosthesis and properties, usage of very long sintering times of 12 hours is carried out, and this increases the patients' treatment plan and inhibits any direct applications [17]. Regarding the response of change in sintering time on the marginal fit of zirconia, evaluated by two studies, the results were somewhat similar. Khaledi Amir et al. [11] found no significant deviation in the marginal fit among 3Y-TZP copings, and all marginal differences were in the clinically acceptable range. A factor that can affect the marginal adaptation is the shrinkage process that occurs during sintering. The sintering parameters, type of material and its density, in turn, determine the shrinkage process. Thus the result can be explained that the three zirconia copings' used in the study had high strength, good density, and stability, and therefore led to insignificance in marginal discrepancy. The sintering time used in the study was 75 minutes, which is quite low compared to the other two sintering times, and the marginal discrepancy was still within clinical range.

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13 The second study by Walaa Magdy Ahmed et al. [14] also studied the influence of change in sintering parameters on the marginal fit of zirconia crowns. The study showed that group 8, which underwent fast sintering, and had a chamfer of 0.5 mm and 1.5 mm thick, had the highest marginal gaps. Most zirconia crowns that underwent the fast sintering process had higher marginal gap values when compared to the standard sintering groups. An exception, however, is the group that had thinner crowns with half a millimeter finish line. The creep of zirconia depends on the temperature; therefore the change in sintering temperature from 1450°C to 1520°C is a possible explanation for this result [14]. This study demonstrates that there is a slight interaction between the change in sintering parameters and marginal adaptation of zirconia copings. Due to the high variability in materials and methodologies, such as the usage of different methods for fit assessment, it is hard to compare the results between the two studies.

Nuri Murat Ersoy et al. [15], Bogna Stawarczyk et al. [16], and Niwut Juntavee et al. [10] reported that with increased sintering temperature there was also an increase in the flexural strength. A possible explanation for this result is that there is a lower amount of porosity in materials sintered at those temperatures. It causes the material to become more compact with fewer voids, thus leading to higher strength [22]. It is a demonstration that alteration in sintering temperature does affect the flexural strength. However, this does not mean that an increase in the sintering temperature will always have a positive effect on flexural strength because, as Bogna Stawaczyk et al. [16] reported when sintering temperature reached above 1600°C; it caused a reduction in the flexural strength. The optimal temperature for higher flexural strength reported by these three studies ranged between 1550 °C and 1580 °C.

In this review a decrease in sintering temperature had no noticeable influence on the flexural strength of the zirconia specimen. It was examined by Nuri Murat Ersoy et al. [15], who reported no significant increase in flexural strength by usage of temperatures of 1510°C and 1540°C with holding times of 8 hours and 2 hours respectively.

When considering the sintering time, the results differ between the articles. Nuri Murat Ersoy et al. [15] reported the highest flexural strength at a sintering time of 10 minutes, which is the lowest sintering time when compared to the time tested in the other two articles. It shows that the employment of shorter times does not necessarily negatively impact the flexural strength, which is promising because shorter sintering times are more helpful in the fast manufacturing of zirconia

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14 prosthesis. Bogna Stawarczyk et al. [16] and Niwut Juntavee et al. [10] used higher sintering times of 2h and 3h and achieved the highest flexural strength at those times. Niwut Juntavee et al. [10] and Bogna Stawrczyk et al. [16] showed that a higher sintering temperature along with a long duration results in higher flexural strength compared to when sintered at low temperature and short sintering time. Jan Ulrich Jansen et al. [17] studied the influence of high speed sintering on flexural strength of zirconia, and reported no change in flexural strength when sintered at a time of ten minutes. Different zirconia materials were used to be compared with each other and it showed that for the materials ZD (Zolid) and Ht-zirconia (High translucent) that the optimal sintering temperature, for the highest flexural strength, was 1590°C. This temperature does not differ much from the before mentioned optimal temperatures reported in the other studies. However, it was also reported by Jan Ulrich Jansen et al. that for the material ZI, there was no noticeable increase in flexural strength by changing the sintering protocols, unlike the other materials. This study confirms that faster sintering protocol with higher temperature doesn’t affect the mechanical properties of zirconia and doesn’t necessarily lead to lower flexural strength.

In contrast to the before mentioned studies, three studies by Kátia Vieira Cardoso et al. [18], Kamal Ebeid et al. [19] and Caner Özturk et al. [20] reported that there was no significant increase in strength when changing sintering protocols. Factors that could have influenced the result are the transformation phases of materials and the different materials used in the studies. Another factor is the difference in the temperature range used in the last three studies in comparison to the other studies. Katia Vieira Cardoso et al. [18] used sintering protocols of 1450°C and 1600°C. Kamal Ebeid et al. [19] used three sintering temperatures in the study: 1460 °C, 1530°C, and 1600°C. Nuri Murat Ersoy et al. [15] reported that sintering temperatures of 1510 °C and 1540°C were too low to cause any change in flexural strength, which could be a possible explanation for the insignificance in flexural strength change in those two studies.

Flexural strength change occurred at temperatures of 1550°C and 1580°C, as was reported by the three authors Nuri Murat Ersoy et al. [15], Bogna Stawarczyk [16] and Niwut Juntavee et al. [10]. Thus the difference in results could possibly be due to the fact that the temperatures used in those three studies were more optimal than what was used in the studies by Kamal Ebeid et al. [19], Katia

Vieira Cardoso et al. [18] and Caner Öztrurk et al. [20]. In addition to that, as was mentioned

before, when sintering temperature reached above 1650°C, there was a decrease in the flexural strength. It was noted that when the zirconia specimen were sintered at high temperatures, a defect in the microstructure could be seen, which causes the decrease in strength [16].

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3.1 Limitations to this study

Only two studies included the influence of changes in the sintering parameters on the marginal accuracy of zirconia copings, and this is a limitation due to the inadequate number of studies found about this topic. Insufficiency in studies concerning the effect on flexural strength, caused only seven studies to be included. The results are therefore limited and established by the limited data of only nine studies. The articles studying the change in sintering parameters on flexural strength used different sintering times and temperature ranges, this makes it challenging to compare articles with each other. The method of flexural strength testing also differed in some of the studies. Another difference is the used Zirconia brands; it was not similar brands in all articles. The effect of sintering time on the flexural strength is not tested in many articles, and therefore, limited comparison between the articles is achieved.

CONCLUSION

1. From the two articles included, we can conclude that there was no significant difference in marginal adaptation when changing sintering parameters. Application of shorter sintering time is acceptable since the marginal discrepancy values were within the clinically acceptable range in the included studies.

2. Sintering temperatures between 1550°C and 1580°C caused an increase in flexural strength. However, temperatures above 1600°C caused a decrease in flexural strength.

3. Shorter sintering times did not cause a change in the flexural strength of zirconia material, and can, therefore, be utilized in dental practices.

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PRACTICAL RECOMMENDATIONS

The recommendations after reading the articles are the following:

1. The usage of higher temperature leads to some increase in flexural strength.

2. Shorter sintering times can be utilized since they do not lead to high marginal discrepancies. 3. Sintering temperature should be kept at certain degrees, preferably not above 1580°C;

otherwise, damage to the microstructure of the specimen can occur.

4. Higher sintering temperature, in combination with short sintering time, does not cause a decrease in flexural strength but can increase it.

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CHAPTER V ANNEXES

Annex No. 1

INDIVIDUAL DEVELOPMENT PLAN FOR THE MASTER’S THESIS

Graduate student

___________________________________________________________________,

of the year ______, and the group _____ of the integrated study programme of Odontology

Duration of studies from __________________till_________________

Supervisor ____________________________________________________________________ MT title: _____________________________________________________________________ ______________________________________________________________________________ MT annotation: ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ Aim of the work:

______________________________________________________________________________ ______________________________________________________________________________ Tasks of the work

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______________________________________________________________________________ ______________________________________________________________________________ _____________________________________________________________________________

Schedule of the works

Performance

Done/not done

No. Description of MT task (supervisor’s evaluation

deadline

and signature)

Graduate student’s name, surname and signature ________________________________

SINTERING EFFECT ON THE MARGINAL FIT AND FLEXURAL STRENGTH OF ZIRCONIA CROWNS: SYSTEMATIC REVIEW OF LITERATURE

Balsem Ahmed Sabah