in the last few years , numerous new dental restorative materials have been introduced in 
 response to an increasing demand for esthetic and biocompatible materials .
the high - strength all - ceramic materials that are currently used in dentistry consist of 
 alumina , zirconia , pressed , castable or machinable glass ceramics .
several developments 
 have taken place in these areas resulting in the production of ceramic materials for 
 clinical use .
these include the aluminous porcelain crown ( vitadur , vita zahnfabrik , bad 
 sckingen , baden - wrttemberg , germany ) , the non shrink ceramic crown ( cerestore , johnson 
 and johnson , east windsor , nj , usa ) , the castable mica glass - ceramic crown ( dicor , 
 caulk / dentsply , milford , de , usa ) and the leucite - reinforced glass ceramics ( ips 
 empress , ivoclar vivadent , schaan , liechtenstein ) . all these all - ceramic systems exhibit 
 low flexural strengths ( 100 - 150 mpa ) , which make them at risk of failure when used for 
 the construction of either posterior crowns or fixed partial dentures . due to the relatively low strength of the early types of ceramics employed in the 
 conventional porcelain jacket crowns ,
an alumina - reinforced porcelain core material was 
 developed by mclean for the fabrication of such crowns .
a veneer porcelain placed on a core containing 
 approximately 50% fused alumina crystals , compared to the conventional feldspathic 
 porcelain level of about 19% , resulted in a dental ceramic with flexural strength from 
 100 to 130 mpa
. the popularity of high - strength ceramic systems is increasing , and the range of their 
 clinical indications is expanding constantly . lithium disilicate ceramics ( ips empress 
 2 , ivoclar vivadent , schaan , liechtenstein ) , infiltrated alumina ceramic ( in - ceram 
 alumina , vita zahnfabrik , bad sckingen , baden - wrttemberg , germany ; turkom - cera fused 
 alumina , turkom - ceramic ( m ) sdn .
puchong , selangor , malaysia ) , densely sintered 
 aluminum oxide ceramic ( procera ) , and zirconium oxide ceramic ( procera allzirkon , nobel 
 biocare , gteborg , sweden ; lava , 3 m espe , st .
paul , mn , usa ) are popular high - strength 
 ceramic materials that offer favorable esthetic characteristics , mechanical properties 
 and biocompatibility .
in - ceram alumina has a high strength ceramic core fabricated through the slip - casting 
 technique .
a slurry of densely packed al2o3 
 ( 8082 wt% ) is applied and sintered to a refractory die at 1120c for 10 h. this 
 produces a porous skeleton of alumina particles which is infiltrated with a 
 low - viscosity glass in a second firing at 1100c for 4 h. this system is conventionally 
 used as a core material in conjunction with a more translucent ceramic to enhance 
 esthetic properties .
advances in dental ceramics include the introduction of a high - strength all - ceramic core 
 material ( turkom - cera ) containing primarily aluminum oxide ( 99.98% ) .
a stone die is 
 covered by a 0.1-mm - thick red plastic foil and dipped in the turkom - cera alumina gel 
 ( 99.98% ) following the manufacturer 's instructions . after drying of the alumina gel , the 
 coping with the red plastic foil is removed from the stone die and sintered for 5 min at 
 1150c .
the sintered coping is crystal hardened in a second firing process using 
 turkom - cera crystal powder for 30 min at 1150c .
like all other infiltration ceramics , 
 this core is then veneered with porcelain adjusted to have the correct coefficient of 
 thermal expansion ( 6.5 - 7.2 ) .
many ceramics are currently available and marketed for use as dental crown and bridge 
 materials .
it has not been ascertain whether the properties of the newer dental 
 materials enable their clinical use to be extended to crowns and bridges in the 
 posterior region .
the maximum biting forces that may occur in the posterior area vary 
 between 300 and 880 n. therefore , it 
 is important for the posterior restorations to be able to withstand the maximum biting 
 forces created in this region .
although long - term clinical studies constitute the ultimate basis on which to reliably 
 predict the long - term performance of such restorations , several physical and mechanical 
 properties are essential to support the correct indication of these materials . because of
different compositions and 
 manufacturing techniques , dental ceramics vary in their physical and mechanical 
 properties .
one important 
 property is the strength of the materials , and specially the flexural strength , because 
 of the brittle nature of ceramics . 

therefore , the aim of this study was to evaluate the flexural strength of turkom - cera 
 compared to two other all - ceramic materials .
three different types of ceramic materials , turkom - cera [ ( turkom - ceramic ( m ) sdn . 

puchong , selangor , malaysia ) ] , in - ceram ( vita zahnfabrik , bad sckingen , 
 baden - wrttemberg , germany ) and vitadur - n ( vita zahnfabrik ) were used in this 
 study .
perspex split mold with five circular openings of 16 mm diameter and 2 mm thickness 
 was used for the preparation of the turkom - cera disc specimens .
the turkom - cera 
 alumina gel was mixed to an optimum consistency and placed into the disc - shaped 
 perspex mold .
the turkom - cera alumina gel was left in the mold for 24 h. after drying 
 of the alumina gel , the discs were taken from the mold and fired ( sintered ) in the 
 furnace ( ivoclar vivadent , programat p300 , ivoclar vivadent ag , schaan , 
 liechtenstein ) for 5 min at 1150c .
the turkom - cera crystal powder was mixed with 
 water and the sintered discs were crystal - hardened in a second firing process in the 
 same furnace for 30 min at 1150c .
a total of 10 turkom - cera discs ( 16 mm diameter and 2 mm 
 thickness ) were fabricated .
perspex split mold with an open top and bottom 5 circular openings ( 16 mm diameter 
 and 2 mm thickness ) was used for the preparation of the in - ceram disc specimens .
the 
 mold was rested and secured on a base made from gypsum die material ( densite , shofu 
 inc . ,
the in - ceram alumina slip was prepared by mixing in - ceram 
 alumina powder with in - ceram mixing fluid and additive supplied by the manufacturer . 

the slip was poured into the mold and dried for 24 h. after drying , the in - ceram 
 alumina discs were taken from the mold and fired using the in - ceramat furnace ( vita 
 zahnfabrik ) for 6 h at 120c and 4 h at 1120c .
the in - ceram glass powder was mixed 
 with water and the sintered in - ceram alumina discs were glass - infiltrated in a second 
 firing process in the same furnace for 30 min at 200c and 4 h at 1100c .
a total of 10 in - ceram discs ( 16 mm diameter 
 and 2 mm thickness ) were fabricated .
according to the results of a preliminary study , vitadur - n porcelain discs of initial 
 diameter 18 mm shrunk to 15.5 - 16 mm in diameter when fired .
therefore , a brass split 
 mold with five circular openings of 18 mm diameter was used for the preparation of 
 the vitadur - n disc specimens .
vitadur - n aluminous core porcelain powder was mixed 
 with vita modeling liquid p to an optimum slurry consistency .
the slurry was placed 
 into the disc - shaped brass mold and vibrated to reduce air bubbles .
a brass compactor 
 was also machined and used to condense the slurry into the mold in order to obtain 
 flat surface .
the condensed slurry was left in the mold for 30 min and excess liquid 
 was blotted away with absorbent tissue .
a layer of vita modisol ( vita zahnfabrik ) 
 separating medium was applied to the mold before the porcelain mixture was poured to 
 facilitate removal of the set porcelain without any distortion .
the disc specimens 
 were then fired according to the manufacturer 's recommendation in a multimat - touch 
 vacuum furnace ( dentsply , dreieich , hessen , germany ) .
the furnace was programmed to 
 give a temperature of 1120c for 60 s under vacuum followed by a further 60 s at 
 atmospheric pressure .
a total of 10 vitadur - n aluminous core porcelain discs with 
 15.5 - 16 mm diameter were fabricated . in order to meet the exact requirements of the biaxial testing protocol
recommended 
 by iso ( 1995 ) , all specimens were 
 subsequently grinded to a parallel shape using the grinder / polisher machine ( metaserv 
 2000 , buehler , coventry , west midlands , uk ) .
a custom made specimen holder made from 
 aluminum was designed and used for the grinding purpose .
eight specimens were fixed 
 into the specimen holder using modeling wax ( figure 
 1 ) .
the initial grinding was performed under running water using a diamond 
 grinding disc with a grain size of 70 m , followed by fine - grinding using a grain 
 size of 30 m .
after that , the specimens were polished with a 15 m diamond polishing 
 paste on a polishing cloth for two min .
they were then rinsed thoroughly with running 
 water for 20 s and dried in air .
specimens mounted in the specimen holder in compliance with iso ( 1995 ) , 
 the specimens were trimmed to 1.20.2 mm in thickness with parallelism of 0.05 mm 
 measured using the digital caliper ( mitutoyo corp , tokyo , japan ) .
piston - on - three - ball test was used for the testing . in order to carry out the test , a 
 loading pin and mounting jig
were designed and used with the instron testing machine 
 ( instron 4302 , instron corporation , england ) .
the mounting jig had a circular opening of 16 mm in 
 diameter with three depressions positioned at equal distances from each other ( 120 
 apart ) and 5 mm from the center forming a tripod .
these depressions were the sites 
 for the 3.2 mm stainless steel ball bearing supports .
the specimens were placed in the mounting jig which ensured the same relation between 
 the supports and the applied load for all specimens .
the 1.6 mm diameter loading pin 
 was mounted to the crosshead of the instron testing machine and applied the load at 
 the center of each specimen ( figure 2 ) .
the 
 test was carried out at a crosshead speed of 0.5 mm / min .
the definitive fracture load 
 was recorded for each specimen and the biaxial flexural strength was calculated from 
 the following equation : biaxial 
 flexural strength=-0.238 7p(x - y)/d ; 
 x=(1+v)ln(r2/r3)+[(1-v)/2](r2/r3 ) ; 
 y=(1+v)[1+ln(r1/r3)2]+(1-v)(r1/r3)2 , 
 where p is the total load causing fracture ( n ) , v is poisson 's ratio ( 0.25 ) , r1 
 is the radius of the support circle ( 5.0 mm ) , r2 is the radius of 
 the loaded area ( 0.8 mm ) , r3 is the radius of the specimen ( 8 mm ) , d is 
 the specimen thickness at the origin of fracture ( mm ) .
specimen during testing in the universal testing machine the results of the study were statistically analyzed with the spss software ( v. 
 11.5.0 for windows , spss , chicago , il , usa ) using levene 's test and dunnett 's t3 
 post - hoc test at a preset significance level of 5% to determine 
 if significant differences between tests groups were related to the ceramic material 
 used for each group .
three different types of ceramic materials , turkom - cera [ ( turkom - ceramic ( m ) sdn . 

puchong , selangor , malaysia ) ] , in - ceram ( vita zahnfabrik , bad sckingen , 
 baden - wrttemberg , germany ) and vitadur - n ( vita zahnfabrik ) were used in this 
 study .
perspex split mold with five circular openings of 16 mm diameter and 2 mm thickness 
 was used for the preparation of the turkom - cera disc specimens .
the turkom - cera 
 alumina gel was mixed to an optimum consistency and placed into the disc - shaped 
 perspex mold .
the turkom - cera alumina gel was left in the mold for 24 h. after drying 
 of the alumina gel , the discs were taken from the mold and fired ( sintered ) in the 
 furnace ( ivoclar vivadent , programat p300 , ivoclar vivadent ag , schaan , 
 liechtenstein ) for 5 min at 1150c .
the turkom - cera crystal powder was mixed with 
 water and the sintered discs were crystal - hardened in a second firing process in the 
 same furnace for 30 min at 1150c .
a total of 10 turkom - cera discs ( 16 mm diameter and 2 mm 
 thickness ) were fabricated .
perspex split mold with an open top and bottom 5 circular openings ( 16 mm diameter 
 and 2 mm thickness ) was used for the preparation of the in - ceram disc specimens .
the 
 mold was rested and secured on a base made from gypsum die material ( densite , shofu 
 inc . ,
the in - ceram alumina slip was prepared by mixing in - ceram 
 alumina powder with in - ceram mixing fluid and additive supplied by the manufacturer . 

the slip was poured into the mold and dried for 24 h. after drying , the in - ceram 
 alumina discs were taken from the mold and fired using the in - ceramat furnace ( vita 
 zahnfabrik ) for 6 h at 120c and 4 h at 1120c .
the in - ceram glass powder was mixed 
 with water and the sintered in - ceram alumina discs were glass - infiltrated in a second 
 firing process in the same furnace for 30 min at 200c and 4 h at 1100c .
a total of 10 in - ceram discs ( 16 mm diameter 
 and 2 mm thickness ) were fabricated .
according to the results of a preliminary study , vitadur - n porcelain discs of initial 
 diameter 18 mm shrunk to 15.5 - 16 mm in diameter when fired .
therefore , a brass split 
 mold with five circular openings of 18 mm diameter was used for the preparation of 
 the vitadur - n disc specimens .
vitadur - n aluminous core porcelain powder was mixed 
 with vita modeling liquid p to an optimum slurry consistency .
the slurry was placed 
 into the disc - shaped brass mold and vibrated to reduce air bubbles .
a brass compactor 
 was also machined and used to condense the slurry into the mold in order to obtain 
 flat surface .
the condensed slurry was left in the mold for 30 min and excess liquid 
 was blotted away with absorbent tissue .
a layer of vita modisol ( vita zahnfabrik ) 
 separating medium was applied to the mold before the porcelain mixture was poured to 
 facilitate removal of the set porcelain without any distortion .
the disc specimens 
 were then fired according to the manufacturer 's recommendation in a multimat - touch 
 vacuum furnace ( dentsply , dreieich , hessen , germany ) . the furnace was programmed to 
 give a temperature of 1120c for 60 s under vacuum followed by a further 60 s at 
 atmospheric pressure .
a total of 10 vitadur - n aluminous core porcelain discs with 
 15.5 - 16 mm diameter were fabricated .
in order to meet the exact requirements of the biaxial testing protocol recommended 
 by iso ( 1995 ) , all specimens were 
 subsequently grinded to a parallel shape using the grinder / polisher machine ( metaserv 
 2000 , buehler , coventry , west midlands , uk ) .
a custom made specimen holder made from 
 aluminum was designed and used for the grinding purpose .
eight specimens were fixed 
 into the specimen holder using modeling wax ( figure 
 1 ) .
the initial grinding was performed under running water using a diamond 
 grinding disc with a grain size of 70 m , followed by fine - grinding using a grain 
 size of 30 m .
after that , the specimens were polished with a 15 m diamond polishing 
 paste on a polishing cloth for two min .
they were then rinsed thoroughly with running 
 water for 20 s and dried in air .
specimens mounted in the specimen holder in compliance with iso ( 1995 ) , 
 the specimens were trimmed to 1.20.2 mm in thickness with parallelism of 0.05 mm 
 measured using the digital caliper ( mitutoyo corp , tokyo , japan ) .
piston - on - three - ball test was used for the testing . in order to carry out the test ,
a 
 loading pin and mounting jig were designed and used with the instron testing machine 
 ( instron 4302 , instron corporation , england ) .
the mounting jig had a circular opening of 16 mm in 
 diameter with three depressions positioned at equal distances from each other ( 120 
 apart ) and 5 mm from the center forming a tripod .
these depressions were the sites 
 for the 3.2 mm stainless steel ball bearing supports .
the specimens were placed in the mounting jig which ensured the same relation between 
 the supports and the applied load for all specimens .
the 1.6 mm diameter loading pin 
 was mounted to the crosshead of the instron testing machine and applied the load at 
 the center of each specimen ( figure 2 ) .
the 
 test was carried out at a crosshead speed of 0.5 mm / min .
the definitive fracture load 
 was recorded for each specimen and the biaxial flexural strength was calculated from 
 the following equation : biaxial 
 flexural strength=-0.238 7p(x - y)/d ; 
 x=(1+v)ln(r2/r3)+[(1-v)/2](r2/r3 ) ; 
 y=(1+v)[1+ln(r1/r3)2]+(1-v)(r1/r3)2 , 
 where p is the total load causing fracture ( n ) , v is poisson 's ratio ( 0.25 ) , r1 
 is the radius of the support circle ( 5.0 mm ) , r2 is the radius of 
 the loaded area ( 0.8 mm ) , r3 is the radius of the specimen ( 8 mm ) , d is 
 the specimen thickness at the origin of fracture ( mm ) .
the results of the study were statistically analyzed with the spss software ( v. 
 11.5.0 for windows , spss , chicago , il , usa ) using levene 's test and dunnett 's t3 
 post - hoc test at a preset significance level of 5% to determine 
 if significant differences between tests groups were related to the ceramic material 
 used for each group .
the objective was to test if the mean biaxial flexural strengths of turkom - cera , 
 in - ceram and vitadur - n differ from each other .
the mean biaxial flexural strength and 
 standard deviation of ten specimens were calculated for each of the three groups tested 
 ( table 1 ) .
because of violation of the 
 assumption of homogeneous variances ( levene 's statistic=13.212 , p<0.05 ) , multiple 
 comparisons were performed with dunnett 's t3 post - hoc test at a pre - set 
 significance level of 5% ( table 2 ) . mean and standard deviation ( sd ) of biaxial flexural strength of the three 
 groups multiple comparisons between the three all - ceramic systems
tested based on observed means , the mean difference is significant at the 0.05 
 level the mean biaxial flexural strength values for turkom - cera ( 506.887.01 mpa ) , in - ceram 
 ( 347.428.83 mpa ) and vitadur - n ( 128.712.72 mpa ) differed significantly from each other 
 ( table 2 ) ( p<0.05 ) .
strength is an important mechanical property that can assist in predicting the 
 performance of brittle materials . 
 the uniaxial flexural strength tests , including three - point , and four - point bending 
 tests , and biaxial bending tests are the most commonly applied methods for evaluating 
 the strength of dental restorations . for uniaxial flexural 
 strength tests , the principal stress on the lower surfaces of the specimens is tensile , 
 and it is usually responsible for crack initiation in brittle materials .
however , 
 undesirable edge fracture , which can increase the variance of the failure stress value , 
 might occur .
the method adapted in this study was the one recommended by iso(1995 ) since the test standardizes 
 specimen thickness , diameter , shape and roughness .
in addition , the measurement of the 
 strength of brittle materials under biaxial flexural strength conditions rather than 
 uniaxial flexural strength is often considered more reliable because the maximum tensile 
 stresses occur within the central loading area and edge failures are eliminated . 

therefore , the biaxial flexural test gave less variation in the strength data . according to iso(1995 ) , the biaxial 
 flexural strength is determined by support of a disc specimen on three metal spheres 
 positioned at equal distances from each other and from the center of the disc .
the load 
 is applied to the center of the opposite surface by a flat piston .
furthermore , the flat surface 
 of the test specimen can be easily controlled by conventional metallographic polishing 
 methods and typical dental finishing techniques .
different researchers have studied the biaxial flexural strength of in - ceram core using 
 the same methods as the current study .
the biaxial flexural strength of in - ceram has 
 been found to be 337.5 mpa on the average .
the mean biaxial flexural strength value for in - ceram 
 ( 347.4 mpa ) obtained in the present study is in agreement with this result .
the biaxial flexural strength of vitadur - n core material has been investigated using the 
 same methods as the current study and found to vary from 141.2 to 155 mpa .
the mean biaxial 
 flexural strength value for vitadur - n ( 128.7 mpa ) achieved in the current study is in 
 agreement with these results .
the higher flexural strength obtained with turkom - cera and in - ceram may be attributed to 
 the followings : 1 .
decrease of the total porosity by initial firing ( sintering ) of 
 turkom - cera alumina gel and in - ceram alumina slip ; 2 .
the alumina particles increase the 
 strength of the material and limit potential sites for crack propagation ; 3 .
prevention 
 of the growth of cracks by crack bridging , as the crystals and glass powders in 
 combination with alumina may bridge the opening created by a crack after the crack front 
 passes ; 4 .
compressive stresses , which further improve the strength , are also introduced 
 due to the differences in the coefficient of thermal expansion of the alumina and 
 crystals / glass . despite the high strength reported with high alumina - based ceramics , they are 
 susceptible to fatigue failure that can considerably reduce their strength over time . in 
 this study ,
therefore , 
 further studies are highly recommended to evaluate the fracture analysis and fatigue 
 behavior of new dental ceramics .
in this study , the biaxial flexural strength of three all - ceramic core materials was 
 tested in vitro .
the new high - strength all - ceramic core material 
 containing primarily aluminum oxide had significantly higher flexural strength 
 ( 506.887.01 mpa ) than the other ceramic core materials ( in - ceram : 347.428.83 mpa and 
 vitadur n : 128.712.72 mpa ) .
this study was supported by a grant ( p019/2006c ) ; university of malaya , kuala lumpur , 
 malaysia .
the authors thank turkom - ceramic ( m ) sdn bhd , puchong , selangor , malaysia 
 for supplying the materials used for the preparation of turkom - cera specimens .
 advances in all - ceramic systems have established predictable means of providing 
 metal - free aesthetic and biocompatible materials . 
 these materials must have 
 sufficient strength to be a practical treatment alternative for the fabrication of 
 crowns and fixed partial dentures.objectivesthe aim of this study was to compare the biaxial flexural strength of three core 
 ceramic materials . 
 material and methodsthree groups of 10 disc - shaped specimens ( 16 mm diameter x 1.2 mm thickness - in 
 accordance with iso-6872 , 1995 ) were made from the following ceramic materials : 
 turkom - cera fused alumina [ ( turkom - ceramic ( m ) sdn bhd , puchong , selangor , 
 malaysia ) ] , in - ceram ( vita zahnfabrik , bad sckingen , baden - wrttemberg , germany ) 
 and 
 vitadur - n ( vita zahnfabrik , bad sckingen , baden - wrttemberg , germany ) , which 
 were sintered according to the manufacturer 's recommendations . 
 the specimens were 
 subjected to biaxial flexural strength test in a universal testing machine at a 
 crosshead speed of 0.5 mm / min . 
 the definitive fracture load was recorded for each 
 specimen and the biaxial flexural strength was calculated from an equation in 
 accordance with iso-6872 . 
 resultsthe mean biaxial flexural strength values were : turkom - cera : 506.887.01 mpa , 
 in - ceram : 347.428.83 mpa and vitadur - n : 128.712.72 mpa . 
 the results were 
 analyzed by the levene 's test and dunnett 's t3 post - hoc test 
 ( spss software v11.5.0 for windows , spss , chicago , il , usa ) at a preset 
 significance level of 5% because of unequal group variances ( p<0.001 ) . 
 there 
 was statistically significant difference between the three core ceramics 
 ( p<0.05 ) . 
 turkom - cera showed the highest biaxial flexural strength , followed by 
 in - ceram and vitadur - n . 
 conclusionsturkom - cera core had significantly higher flexural strength than in - ceram and 
 vitadur - n ceramic core materials . 
INTRODUCTION
MATERIAL AND METHODS
Materials
Preparation of disc-shaped specimens
Grinding of specimens
Biaxial flexural strength testing
Statistical analysis
RESULTS
DISCUSSION
CONCLUSION
ACKNOWLEDGEMENTS