porcelains are highly esthetic materials extensively used in dentistry to construct 
 various types of restorations and prostheses such as porcelain fused to metal crowns , 
 veneers , inlays , onlays and all ceramic restorations .
they fulfill the esthetic and 
 functional demands of the patients by their superior properties when compared to other 
 restorative materials .
the new glass 
 ceramics ( ips d.sign ; ivoclar vivadent ag , schaan , liechtenstein ) have become popular 
 for porcelain - fused - to - metal restorations .
ips d.sign is a new type feldspathic - based 
 porcelain containing dispersed fluorapatite and leucite crystals in a feldspathic glassy 
 matrix .
the leucite crystals 
 ( <3 m ) present in the ips d.sign porcelain also contribute to the overall 
 strength .
recently , the new 
 all ceramic systems ( ips e.max ; ivoclar vivadent ag , ) have been introduced into the 
 market .
ips e.max ceram is a veneering porcelain of this system which is a 
 feldspathic - based porcelain having a microstructure unlike ips d.sign .
this porcelain 
 only consists of dispersed fluorapatite crystals in a feldspathic glassy matrix ; thus , 
 having a microstructure unlike that of any other commercially available dental 
 porcelains .
fluorapatite 
 crystals , 2 - 5 m in length and 300 nm in diameter of needle - like morphology , are known 
 to be contained in natural bone and teeth .
these very small crystals in dental 
 microstructures result in very special optical properties such as translucence and 
 opalescence , which also result the same properties as in dental restorations . despite the outstanding esthetic quality of the porcelains ,
these characteristics impair their 
 physical properties such as surface roughness , surface hardness , strength 
 and influence the clinical success and failure of porcelain restorations .
crack propagation and degradation of 
 dental porcelains occur when porcelains are exposed to aqueous solutions or erosive 
 agents .
alkaline metal ions are far 
 less stable in the glass phase than in the crystalline phase .
therefore , some alkaline ions in porcelains were leached 
 after being exposed to acidic solutions .
variation in ph , solution chemistry , wear and 
 mechanical load makes the oral cavity a complex environment .
consequences of porcelain degradation 
 include coarseness of exposed surface , promoting plaque accumulation 
 and wear to antagonist materials . 

the potential 
 erosive effect of these acidic food and beverages on enamel occurs primarily by the 
 dissolution of apatite crystals . however , their effect on the porcelain restorations has not been 
 clearly documented .
therefore , the present in vitro study was designed 
 to evaluate changes of the flexural strength and surface of fluorapatite - leucite and 
 fluorapatite porcelains after being exposed to erosive agents ( pineapple juice , green 
 mango juice , coca soft drink , citrate buffer solution and 4% acetic acid ) in cyclic 
 immersion .
the null hypothesis was that there was no significant difference in flexural 
 strength of each type of dental porcelain tested after being exposed to erosive 
 agents .
two commercial dentin shade a3 porcelain powders were used : ips d.sign and ips e.max 
 ceram ( ivoclar vivadent ag ) ( figure 1 ) .
ips 
 d.sign and ips e.max ceram are indicated to be used as veneering porcelain for 
 porcelain fused to metal and all ceramic restorations , respectively .
one hundred and 
 twenty bar specimens from each of the 2 porcelains were fabricated using the 
 26.0x6.0x3.0 mm silicone mold ( provil novo putty ; heraeus kulzer gmbh , d-63450 hanau , 
 germany ) .
the porcelain powders were mixed with deionized water , filled in the 
 silicone mold and condensed with a condenser ( ceramosonic ii ; shofu inc , 
 higashiyama - ku , kyoto , japan ) .
the specimens were then fired according to the 
 manufacturer 's instructions ( table 1 ) .
after 
 firing , the specimens were polished ( model phoenix 4000 ; buehler gmbh , 40599 
 dsseldorf , germany ) under running water using 600- and 1,200-grit silicon carbide 
 paper ( 3 m espe , st .
paul , mn , usa ) to the dimensions of 25.0x5.0x2.0 mm , following 
 the guidelines of the iso 6872 standard .
then , the specimens were ultrasonically cleaned in distilled 
 water for 10 min , and subjected to self - glazing according to the manufacturer s 
 instructions ( table 1 ) .
porcelains used in the present study firing schedules for the porcelains used in the present study the porcelain bars were divided into 8 groups of 15 specimens each .
subsequently , the 
 specimens were alternately immersed in 25 ml of an erosive agent for 30 min and in 25 
 ml of deionized water for 5 min for 7 cycles at 37c .
this amount of erosive agent 
 ( 25 ml ) was a sufficient volume to completely cover the specimen . in order to 
 maintain the original ph level of the erosive agent ,
the same protocol was used with different types of 5 
 erosive solutions included in the study ( citrate buffer solution , pineapple juice , 
 green mango juice , cola soft drink and 4% acetic acid ; see figure 2 ) and deionized water ( control ) .
the specimens immersion 
 protocol simulated an individual eating acidic food , sour fruits and drinks .
seventh group was continuously immersed in 4% acetic acid 
 at 80c for 16 h ( as modified from iso 6872 ) in order to examine the extensive effect which could occur .
a 
 8 group was kept dry at 37c for 245 min in order to compare the 
 effect of moisture condition .
after the immersion sequence was completed , the 
 specimens were rinsed with deionized water , blotted dry and subjected to flexural 
 strength testing .
erosive agents used in the present study the flexural strength was measured with the universal testing machine ( model 
 lrx - plus ; ametek lloyd instruments , farnborough , hampshire , uk ) .
bar - shaped specimens 
 were centered and placed on two steel spheres ( 1.6 mm in diameter ) of a supporter 
 part positioned 12 mm apart from each other .
three point bending tests were carried 
 out using a 250 n load cell at crosshead speed 0.25 mm / min .
the load at failure was 
 recorded in newton and converted to flexural strength in mpa ( 3wl/2bd ; 
 w = failure load , l = span length , b = specimen s width , and d = specimen s thickness ) .
two - way anova was analyzed to measure statistically significant differences among the 
 types of erosive agents and the type of porcelains after being exposed to erosive 
 agents .
tukey s honestly significant difference ( hsd ) tests were used for 
 post hoc comparisons ( =0.05 ) .
the t - test was 
 used for comparing the flexural strength between the two types of porcelain for each 
 erosive agent ( =0.05 ) .
two commercial dentin shade a3 porcelain powders were used : ips d.sign and ips e.max 
 ceram ( ivoclar vivadent ag ) ( figure 1 ) .
ips 
 d.sign and ips e.max ceram are indicated to be used as veneering porcelain for 
 porcelain fused to metal and all ceramic restorations , respectively .
one hundred and 
 twenty bar specimens from each of the 2 porcelains were fabricated using the 
 26.0x6.0x3.0 mm silicone mold ( provil novo putty ; heraeus kulzer gmbh , d-63450 hanau , 
 germany ) .
the porcelain powders were mixed with deionized water , filled in the 
 silicone mold and condensed with a condenser ( ceramosonic ii ; shofu inc , 
 higashiyama - ku , kyoto , japan ) .
the specimens were then fired according to the 
 manufacturer 's instructions ( table 1 ) .
after 
 firing , the specimens were polished ( model phoenix 4000 ; buehler gmbh , 40599 
 dsseldorf , germany ) under running water using 600- and 1,200-grit silicon carbide 
 paper ( 3 m espe , st .
paul , mn , usa ) to the dimensions of 25.0x5.0x2.0 mm , following 
 the guidelines of the iso 6872 standard .
then , the specimens were ultrasonically cleaned in distilled 
 water for 10 min , and subjected to self - glazing according to the manufacturer s 
 instructions ( table 1 ) .
porcelains used in the present study firing schedules for the porcelains used in the present study
subsequently , the 
 specimens were alternately immersed in 25 ml of an erosive agent for 30 min and in 25 
 ml of deionized water for 5 min for 7 cycles at 37c .
this amount of erosive agent 
 ( 25 ml ) was a sufficient volume to completely cover the specimen . in order to 
 maintain the original ph level of the erosive agent ,
the same protocol was used with different types of 5 
 erosive solutions included in the study ( citrate buffer solution , pineapple juice , 
 green mango juice , cola soft drink and 4% acetic acid ; see figure 2 ) and deionized water ( control ) .
the specimens immersion 
 protocol simulated an individual eating acidic food , sour fruits and drinks .
seventh group was continuously immersed in 4% acetic acid 
 at 80c for 16 h ( as modified from iso 6872 ) in order to examine the extensive effect which could occur .
a 
 8 group was kept dry at 37c for 245 min in order to compare the 
 effect of moisture condition .
after the immersion sequence was completed , the 
 specimens were rinsed with deionized water , blotted dry and subjected to flexural 
 strength testing .
the flexural strength was measured with the universal testing machine ( model 
 lrx - plus ; ametek lloyd instruments , farnborough , hampshire , uk ) .
bar - shaped specimens 
 were centered and placed on two steel spheres ( 1.6 mm in diameter ) of a supporter 
 part positioned 12 mm apart from each other .
three point bending tests were carried 
 out using a 250 n load cell at crosshead speed 0.25 mm / min .
the load at failure was 
 recorded in newton and converted to flexural strength in mpa ( 3wl/2bd ; 
 w = failure load , l = span length , b = specimen s width , and d = specimen s thickness ) .
two - way anova was analyzed to measure statistically significant differences among the 
 types of erosive agents and the type of porcelains after being exposed to erosive 
 agents .
tukey s honestly significant difference ( hsd ) tests were used for 
 post hoc comparisons ( =0.05 ) .
the t - test was 
 used for comparing the flexural strength between the two types of porcelain for each 
 erosive agent ( =0.05 ) .
the flexural strength values of the two types of porcelain were showed in table 2 .
anova results showed that the interaction 
 between the two variables ( type of porcelain and erosive 
 agent ) found statistically significant difference ( p=0.02 ) . between the two 
 dental porcelains ,
a statistically significant difference was also found ( p=0.01 ) , but 
 none was found among the types of erosive agents ( p=0.46 ) .
mean flexural strength  standard deviations ( sd ) of ips d.sign and ips e.max 
 ceram same superscript letters in columns indicate no significant difference ( tukey 's 
 hsd test ; =0.05 ) .
when comparing the mean flexural strength values between the porcelains for each group , 
 the results of the t - test showed that all ips e.max ceram groups 
 yielded a significantly higher mean flexural strength ( p<0.05 ) than that of ips 
 d.sign groups .
the results of this study support acceptance of the null hypothesis , as the flexural 
 strength of both porcelains was not affected by the erosive agents .
it is noticed that 
 the highest flexural strength was found in the dry condition group ( stored at 37c ) of 
 both porcelains and decreased in all groups when the porcelains were immersed in erosive 
 agents as well as in water . the possible explanation for these results could be the 
 effect of glazing in determining flexural strength values .
porcelains , by nature , exhibit inherent flaws or defects on their surface and internal 
 body .
these flaws could impair their physical properties . however , the surface flaws are 
 covered by the glazes , either self - glazing or overglazing .
re - firing the porcelain prior 
 to final restoration produces a self - glaze layer .
this layer may increase the strength 
 of the porcelain restoration from two possible mechanisms .
firstly , when the restoration is heated , the self - glaze 
 layer fills in surface flaws , reducing their depth and blunting the flaw tips .
this 
 should increase strength because , for given porcelains , strength increases with 
 decreasing sharpness and flaw depth .
secondly , for feldspathic - based porcelains , the 
 self - glaze layer has a lower coefficient of thermal expansion than the leucite - rich 
 interior .
the compressive 
 stress state diminishes the local tensile stress produced from applied loading at 
 surface flaws , thereby needing application of increased load to initiate flaw 
 propagation from the external surface .
the ips e.max ceram had higher flexural strength than the ips d.sign in all groups . a 
 possible explanation for this result could be the microstructure of these 
 porcelains .
the ips d.sign , 
 feldspathic - based porcelain , is unique and distinct from other porcelains since its 
 microstructure consists of fluorapatite crystal phases in addition to having leucite 
 particles in a feldspathic glassy matrix , while the ips e.max ceram consists of only dispersed fluorapatite 
 crystals in a feldspathic glassy matrix . in feldspathic - based porcelains ,
the leucite particles contract 
 more than the surrounding glass upon cooling . above a critical particle size ,
the 
 stresses created during cooling can induce microcracks circumferential to the leucite 
 particles .
previous studies 
 have documented that the size of leucite particles in feldspathic porcelain increases 
 during heat treatment within the normal porcelain firing range .
in contrast to fluorapatite porcelains , the fluorapatite phase particles are 
 needle - like and contribute to high flexural strength as well as high chemical 
 durability .
the erosive agents used in this present study , pineapple juice and green mango juice , 
 are favorite sour fruit juices in many asia countries .
they consist of citric acid and 
 other organic acid , which give an 
 acidic ph .
however , in the present study , these juices did not affect the flexural 
 strength of the tested porcelain after immersion , which do not agree with the findings 
 of previous studies that showed an impact of acidic agents on porcelains .
this study was a 
 short - term experiment and could be the reason to explain why there was no significant 
 difference among the acidic agents and this aspect should be explored .
so , a long - term 
 evaluation of the effect of erosive agents on porcelains is required .
for example , the 
 presence of water , temperature change , the ph level and the role of saliva in the oral cavity may considerably 
 influence strengths of restorations .
in addition , the present study evaluated only 
 fluorapatite - leucite and fluorapatite porcelains .
within the limitations of this study , the flexural strengths of the porcelains 
 ( fluorapatite - leucite and fluorapatite porcelains ) after exposure to erosive agents in 
 cyclic immersion were not significantly different .
for both types of porcelain , dry 
 storage at 37c yielded the highest flexural strength , though without significant 
 difference from the other groups .
the flexural strengths of all groups of fluorapatite 
 porcelains were significantly higher than those of the fluorapatite - leucite 
 porcelains .
 objectivethe aim of this study was to evaluate the flexural strength of two porcelain 
 materials ( ips d.sign and ips e.max ceram ) exposed to erosive agentsmaterial and methodsone hundred and twenty bar - shaped specimens were made from each of 
 fluorapatite - leucite porcelain ( ips d.sign ) and fluorapatite porcelain ( ips e.max 
 ceram ) and divided into 8 groups of 15 specimens each . 
 six groups were alternately 
 immersed in the following storage agents for 30 min : deionized water ( control ) , 
 citrate buffer solution , pineapple juice , green mango juice , cola soft drink and 
 4% acetic acid . 
 then , they were immersed for 5 min in deionized water at 37c . 
 
 seven cycles were completed , totalizing 245 min . 
 a 7th group was 
 continuously immersed in 4% acetic acid at 80c for 16 h. the final , 
 8th , group was stored dry at 37c for 245 min . 
 three - point bending 
 tests were performed in a universal testing machine . 
 the data were analyzed 
 statistically by 2-way anova , tukey s hsd test and t - test at significance level of 
 0.05.resultsthe flexural strengths of all groups of each porcelain after exposure to erosive 
 agents in cyclic immersion did not differ significantly ( p>0.05 ) . for both 
 types of porcelain , dry storage at 37c yielded the highest flexural strength , 
 though without significant difference from the other groups ( p>0.05 ) . 
 the 
 flexural strengths of all groups of fluorapatite porcelains were significantly 
 higher ( p<0.05 ) than those of the fluorapatite - leucite porcelains.conclusionsthis study demonstrated that the erosive agents evaluated did not affect the 
 flexural strength of the tested dental porcelains . 
INTRODUCTION
MATERIAL AND METHODS
Specimen Preparation
Erosive Agents Exposure
Flexural Strength Measurements
Statistical Analysis
RESULTS
DISCUSSION
CONCLUSIONS