metal - ceramic restorations have been successfully applied in restorative dentistry due to high fracture strength . however , achieving a natural translucency is more difficult with metal - ceramic restorations than all - ceramic restorations because metal substructures prevent light transmission in metal - ceramic restorations .
this has led to an increase in use of all - ceramic restorations . among all - ceramic restorations ,
however , high translucency of a restoration is not always an advantage , for instance in cases with discolored teeth , colored core materials and cast metal post and cores . in these clinical conditions , a restorative material with optimal masking ability of the underlying substructures
a method to evaluate the masking ability of restorative materials is to measure color differences in cie lab color system . in this color system ,
the l * , a * and b * color parameters and e refer to lightness , red - green value , yellow - blue value , and color difference , respectively , which can be measured by a spectrophotometer .
the e color difference is calculated using this formula : 
 e*ab=[(l*2l*1)2+(a*2a*1)2+(b*2b*1)2]1/2 
 which is the most commonly used formula for e .
this formula can detect even a small amount of color difference between two objects [ 9 , 10 ] .
as spectrophotometers can detect even the slightest amount of color difference not perceptible by the human eye , perceptual and clinically acceptable thresholds have been defined based on e values .
it has been reported that the clinically acceptable threshold is more than the perceptual threshold , because of optical conditions of oral environment .
if the e ( color difference ) between two objects is more than the perceptual threshold , a color mismatch will be detected by the human eye .
the perceptual threshold has been reported from e=1 to e=5.5 in different studies [ 1214 ] .
if a restorative material has an ideal masking ability , e will be equal to zero when placing the material on white and black substrates , namely that the material has a similar color on different substrates , and the substrates can not affect the color of the material . from a clinical point of view , if e of a material on different backgrounds is less than the threshold , it can thoroughly mask the background color .
several studies have surveyed the optical properties of different ceramic materials [ 8 , 1518 ] .
some studies have evaluated various factors affecting the final color of zirconia restorations including dental substrates [ 1922 ] , luting agents [ 21 , 23 ] , ceramic veneers [ 2426 ] , glaze , and firing process .
suputtamongkol et al ,   reported that the background color could affect the overall color of posterior zirconia restorations on a metal post and core or a prefabricated post and a composite core ; however , color changes did not exceed the perceptual threshold .
choi and razzoog   assessed the masking ability of a zirconia ceramic ( zrc ) with and without the veneering porcelain and revealed that the zirconium oxide coping material alone had some degree of masking ability .
oh and kim   disclosed that abutment shade , ceramic thickness and coping type affected the resulting color of zirconia restorations .
vichi et al ,   evaluated the effects of different substrates with different colors on the final color of all - ceramic ips - empress glass - ceramic restorations , and advised to take the color of substrate into consideration in cases with ceramic thickness of less than 1 mm .
masking ability of a zrc is related to its color coverage over underlying substructures including cement and dental substrates .
it may not be feasible to mask a substrate with cements , because different shades do not exist for all cements .
thus , it seems that the color masking ability of a zrc on different substrates may be an important factor affecting the esthetic results of zirconia - based restorations . on the other hand , posts and cores , which are widely applied in restorative dentistry to restore severely damaged teeth
the effects of these core materials ( as substrates ) on masking ability of zrcs ( as substructures in zirconia - based restorations ) have to be clearly determined . therefore ,
this in vitro study aimed to evaluate the effect of three different core materials including a nickel - chromium alloy ( nca ) , a non - precious gold alloy ( npga ) , and a zrc on masking ability of a zrc . the null hypothesis was that the zrc would mask the three aforementioned core materials with no significant difference .
the discs were placed over four substrates including a white ( w ) substrate ( as the control ) , nca , npga and zrc .
the procedure was conducted as follows : 
 specimen preparation : a computer - aided design / computer - aided manufacturing system ( coritec 250i , imes - icore gmbh , eiterfeld , germany ) milled zirconia blanks ( luminesse high strength / low translucency , 98 mm discs # 5113 ; talladium , valencia , ca , usa ) to fabricate zirconia discs .
the zirconia blanks had 37% translucency and were used to fabricate zirconia frameworks in zirconia - based restorations .
the discs had 0.5 mm thickness and 10 mm diameter [ 20 , 23 ] .
all the zirconia discs were sintered at 1520c for a 12-hour process in a sintering furnace ( isint ht , imes - icore gmbh , eiterfeld , germany ) . a digital micrometer ( 293 mdc - mx lite ; mitutoyo corporation , tokyo , japan ) with the accuracy of 0.002 mm
an adjustment / polishing kit ( bruxzir ; glidewell direct , irvine , ca , usa ) was used to reduce the thicknesses according to the afore - mentioned acceptable range . in case of lack of acceptable thickness
the zirconia discs were polished , cleaned in an ultrasonic bath ( elmasonic s-30 ; dentec , north shore , australia ) containing 98% ethanol for 15 minutes , and were finally dried .
the zirconia disc specimens were prepared as such , and had natural white color of zirconia ( fig .
1).substrate preparation : in order to fabricate the w substrate with the dimensions of 10 mm in diameter and 10 mm in height , a white teflon material ( ptfe , omnia plastica spa , busto arsizio , italy ) was milled .
then , two cylindrical acrylic resin ( duralay , reliance dental mgf co. , worth , il , usa ) patterns were formed according to the afore - mentioned dimensions .
the patterns were cast by a nca ( verabond v ; alba dent , fairfield , ca , usa ) and a npga ( alba dent , fairfield , ca , usa ) to fabricate nca and npga substrates , respectively . an adjusting / polishing kit ( np alloy adjustment kit ; shofu inc . , kyoto , japan )
was employed to polish the alloy substrates . in order to fabricate the zrc substrate ,
a cylindrical pattern was virtually designed by a software program ( solidworks 2015 , solidworks corp . ,
the same computer - aided design / computer - aided manufacturing system milled a zirconia blank ( luminesse high strength 98 mm discs # 5113 ; talladium , valencia , ca , usa ) to fabricate a zirconia substrate based on the above - mentioned virtual design .
the zirconia substrate was dipped in a2 shade color liquid ( dd bio zx2 monolithic zero lzdd ; dental direkt gmbh , spenge , germany ) for 10 seconds .
the zirconia substrate was sintered at 1520c for a 12-hour process in the same sintering furnace , and was then polished by the same polishing kit .
the four substrates of w , nca , npga , and zrc were prepared as such .
all the substrates were cleaned in the same ultrasonic bath containing 98% ethanol for 15 minutes ( fig .
2 ) color measurement : a spectrophotometer ( spectroshade micro , mht , verona , italy ) was employed for spectrometric measurements . a putty silicone material ( speedex ; coltene , altstatten , switzerland )
was adapted to the mouth piece of the spectrophotometer to match the conditions of spectrophotometry for all specimens and to prevent external lights .
the specimens were located at the center of this putty mold . before each measurement ,
the spectro - photometer was calibrated by the white and green calibration plates , respectively .
the discs were placed over the substrates with a water drop in - between to prevent the refraction of light .
each disc was placed on each of the substrates , and the color measurements were made .
all the color measurements were made at the center of specimens marked by a pen on the monitor screen of spectrophotometer .
the color parameters of l * , a * , and b * were recorded for each specimen .
additionally , e values were calculated to determine the e of disc on the substrates . in order to compare the color of specimens on the nca , npga , and
zrc substrates with their color on the w substrate , the e values were measured in three situations including : w - nca , w - npga , and w - zrc .
the e 

 w - nca 

 e 

 w - npga 

 , and e 

 w - zrc 

 values were calculated using this formula : 
 e*ab=[(l*2l*1)2+(a*2a*1)2+(b*2b*1)2]1/2 

  specimen preparation : a computer - aided design / computer - aided manufacturing system ( coritec 250i , imes - icore gmbh , eiterfeld , germany ) milled zirconia blanks ( luminesse high strength / low translucency , 98 mm discs # 5113 ; talladium , valencia , ca , usa ) to fabricate zirconia discs .
the zirconia blanks had 37% translucency and were used to fabricate zirconia frameworks in zirconia - based restorations .
the discs had 0.5 mm thickness and 10 mm diameter [ 20 , 23 ] .
all the zirconia discs were sintered at 1520c for a 12-hour process in a sintering furnace ( isint ht , imes - icore gmbh , eiterfeld , germany ) . a digital micrometer ( 293 mdc - mx lite ; mitutoyo corporation , tokyo , japan ) with the accuracy of 0.002 mm
an adjustment / polishing kit ( bruxzir ; glidewell direct , irvine , ca , usa ) was used to reduce the thicknesses according to the afore - mentioned acceptable range . in case of lack of acceptable thickness
the zirconia discs were polished , cleaned in an ultrasonic bath ( elmasonic s-30 ; dentec , north shore , australia ) containing 98% ethanol for 15 minutes , and were finally dried .
the zirconia disc specimens were prepared as such , and had natural white color of zirconia ( fig .
substrate preparation : in order to fabricate the w substrate with the dimensions of 10 mm in diameter and 10 mm in height , a white teflon material ( ptfe , omnia plastica spa , busto arsizio , italy ) was milled .
then , two cylindrical acrylic resin ( duralay , reliance dental mgf co. , worth , il , usa ) patterns were formed according to the afore - mentioned dimensions .
the patterns were cast by a nca ( verabond v ; alba dent , fairfield , ca , usa ) and a npga ( alba dent , fairfield , ca , usa ) to fabricate nca and npga substrates , respectively . an adjusting / polishing kit ( np alloy adjustment kit ; shofu inc . ,
kyoto , japan ) was employed to polish the alloy substrates . in order to fabricate the zrc substrate ,
a cylindrical pattern was virtually designed by a software program ( solidworks 2015 , solidworks corp . ,
the same computer - aided design / computer - aided manufacturing system milled a zirconia blank ( luminesse high strength 98 mm discs # 5113 ; talladium , valencia , ca , usa ) to fabricate a zirconia substrate based on the above - mentioned virtual design .
the zirconia substrate was dipped in a2 shade color liquid ( dd bio zx2 monolithic zero lzdd ; dental direkt gmbh , spenge , germany ) for 10 seconds .
the zirconia substrate was sintered at 1520c for a 12-hour process in the same sintering furnace , and was then polished by the same polishing kit .
the four substrates of w , nca , npga , and zrc were prepared as such .
all the substrates were cleaned in the same ultrasonic bath containing 98% ethanol for 15 minutes ( fig .
2 ) color measurement : a spectrophotometer ( spectroshade micro , mht , verona , italy ) was employed for spectrometric measurements . a putty silicone material ( speedex ; coltene , altstatten , switzerland )
was adapted to the mouth piece of the spectrophotometer to match the conditions of spectrophotometry for all specimens and to prevent external lights .
the specimens were located at the center of this putty mold . before each measurement ,
the spectro - photometer was calibrated by the white and green calibration plates , respectively .
the discs were placed over the substrates with a water drop in - between to prevent the refraction of light .
each disc was placed on each of the substrates , and the color measurements were made .
all the color measurements were made at the center of specimens marked by a pen on the monitor screen of spectrophotometer .
the color parameters of l * , a * , and b * were recorded for each specimen .
additionally , e values were calculated to determine the e of disc on the substrates . in order to compare the color of specimens on the nca , npga , and
zrc substrates with their color on the w substrate , the e values were measured in three situations including : w - nca , w - npga , and w - zrc .
the e 

 w - nca 

 e 

 w - npga 

 , and e 

 w - zrc 

 values were calculated using this formula : 
 e*ab=[(l*2l*1)2+(a*2a*1)2+(b*2b*1)2]1/2 
  zirconia disc specimens tested substrates ( from left to right : zirconia ceramic , non - precious gold alloy , nickel - chromium alloy and white substrate ) the perceptual threshold of e=2.6 was considered in this study [ 1214 ] .
normal distribution of the data was accepted in all groups by the kolmogorov - smirnov test ( p>0.05 ) .
the values of l * , a*and b * for each disc in npga , nca and zrc groups were subtracted from l*white , a*white and b*white values , which were the corresponding values for w substrate . in order to compare the standard l * , a * , b * ( from which , the values of w substrate were subtracted ) and e values among the groups , randomized block anova was employed .
stata software ( statacorp lp , lakeway , tx , usa ) compared the e values with the predetermined perceptual threshold of e=2.6 using one - sample t - test .
the results were explained according to the measured variables of l * , a * , b * , and e in four sections .
l * parameter ( lightness ) : the mean and standard deviation of the l * values for the w , nca , npga and zrc groups were 88.351.46 , 78.241.31 , 78.981.26 , and 82.161.16 units , respectively ( table 1 , fig .
3 ) . statistical analysis showed a significant difference among the groups in this regard ( p<0.0001 ) .
pairwise comparisons of the groups using bonferroni test revealed significant differences between the nca and zrc ( p<0.0001 ) , and npga and zrc ( p<0.0001 ) .
the color parameter values in the four groups of white substrate ( w ) , nickel - chromium alloy ( nca ) , non - precious gold alloy ( npga ) and zirconia ceramic ( zrc ) the mean l * values of the groups a * parameter ( red - green value ) : the mean and standard deviation of the a * values for the w , nca , npga and zrc groups were 0.400.42 , 0.040.52 , 0.600.42 , and 1.520.38 units , respectively ( table 1 , fig .
pairwise comparisons of the groups using bonferroni test revealed significant differences between nca and npga ( p<0.0001 ) , nca and zrc ( p<0.0001 ) and npga and zrc ( p<0.0001 ) .
the a * values increased in all the groups compared to the control . the increase in the a
the mean a * values of the groups b * parameter ( yellow - blue value ) : the mean and standard deviation of the b * values for the w , nca , npga , and zrc groups were 3.380.36 , 1.921.01 , 3.860.49 , and 4.880.57 units , respectively ( table 1 , fig .
pairwise comparisons of the groups using bonferroni test showed significant differences between nca and npga ( p<0.0001 ) , nca and zrc ( p<0.0001 ) , and npga and zrc ( p<0.001 ) .
zrc had the highest b * value , and nca had the lowest b * value among the groups .
the mean b * values of the groups e ( color difference ) : the mean and standard deviation of the e values for the nca , npga and zrc groups were 10.262.43 , 9.451.74 , and 6.701.91 units , respectively ( table 1 , fig .
pairwise comparisons of the groups using bonferroni test showed significant differences between nca and zrc ( p<0.0001 ) , and npga and zrc ( p=0.001 ) .
the difference between nca and npga was not statistically significant ( p=0.65 ) . in order to compare the e values of the groups with the predetermined perceptual threshold of e=2.6 ,
the null hypothesis of e  2.6 was rejected for all the groups ( p<0.0001 ) .
the present study evaluated the color parameters of l * , a * , b * , and e for zirconia disc specimens on four different substrates including w , nca , npga and zrc .
statistical analysis indicated significant differences among the groups in the l * , a * , b * and e values .
the examined zrc on the tested substrates showed perceptible color change namely that the examined zrc could not thoroughly mask the tested substrates .
the l * ( lightness ) values decreased in all the groups compared with the control .
the change in l * value shows the impact of the substrates on the zirconia specimens .
this can be explained by the optical properties of zrc , which is a semi - translucent material .
according to the results , metal core materials such as ncas and npgas can decrease the lightness of zrc more than a zirconia core material
the a * ( red - green ) values increased in all the groups compared to the control .
a reason for this may be the natural a * value of the zrc as a substrate .
the tested zirconia substrate ( a2 shade ) positively shifted the a * value towards red .
the b * ( yellow - blue ) values changed in the groups depending on the substrates . the nickel - chromium substrate decreased the b * value while the non - precious gold and zirconia substrates increased the b * value compared to the control .
this may be due to the yellow color tendency of a2 shade zirconia substrate and npga , which impact on the color of zrc and shift it towards yellow . on the contrary , nca shifted the color of zrc towards blue , and negatively affected the b * value .
the e values of all groups were more than the predetermined perceptual threshold of e=2.6 .
assessment of the l * , a * and b * values indicated their changes in the substrate groups . thus , color changes were derived from all three color parameters .
the l * , a * , and b * values manifested significant differences between the substrate groups .
this demonstrated that the quality of the color change was different depending on the substrate .
the e values manifested no significant difference between the metal substrates ; however , significant differences existed between the zirconia substrate and the metal substrates , and the least color change belonged to the zirconia substrate .
this demonstrated that the quantity of the color change was different depending on the substrate as well .
hence , both quality and quantity of the color of zrc were altered by the substrates .
the used metal substrates did not differ in terms of the quantity of color change , and both affected the quantity of the color change more than the zirconia substrate .
suputtamongkol et al ,   reported that the color of an underlying substructure could affect the overall color of posterior zirconia - based restorations on a metal post and core or a prefabricated post and a composite core , ranging from e = 1.2 to 3.1 .
additionally , slight color changes of zirconia crowns were detected by measuring the e . although some differences exist between the afore - mentioned study and the current research such as zrc brands , thicknesses of ceramics , veneered versus non - veneered zrcs , hypothesized perceptual thresholds and substrate types , both studies showed that zrc could not thoroughly mask the underlying materials .
oh and kim   in an in vitro study of color masking ability assessed the effects of abutment shade , ceramic thickness and coping type on the final color of zirconia restorations .
the abutments were fabricated of gold alloy , base metal ( nickel - chromium ) alloy , and four different shades of composite resins . in their study , the average e value of lava specimens between the a2 shade composite resin and gold alloy abutments was higher than that between the a2 shade composite resin and other abutments , and was close to e=5.5 .
a similar result was obtained in the current study , although the substrates and zrcs used in the two studies were different .
according to the study by oh and kim , gold alloy substrate yielded the highest e among the tested substrates ; however , the npga and the nca yielded close e values in the current study .
this dissimilarity in the results may be caused by the color difference of the precious gold alloy used by oh and kim   and the npga used in the current study .
choi and razzoog   evaluated the masking ability of zrc with and without a veneering porcelain . in their study , color differences caused by zrc and the veneering porcelain were compared with substrates alone .
they concluded that the non - veneered zrc had some degree of masking ability over different tested substrates ( white , black , gray , and a3 shade tooth - colored substrates ) .
however , the current study compared substrate - induced color differences with a zrc over w substrate and concluded that the tested zrc could not mask the examined substrates .
choi and razzoog   measured the e values between the substrate alone ( as control ) and the zrc over the substrate , while the current study measured the e values between the zrc over w substrate ( as control ) and zrc over the tested substrates .
based on the results of our study , the three core materials including nca , npga and zrc can change the color of tested zrc .
the color of zrc may be closer to the final color of a restoration than the metal cores .
therefore , it is esthetically advised to preferably choose a zrc post and core instead of nca or npga post and cores in zirconia - based restorations and to reduce the core material - induced color change by further core reduction , applying sufficient thickness of the veneering porcelain , and use of proper luting agents in zirconia - based restorations .
it should be noted that given that the physical and mechanical properties of zirconia post and core are confirmed in further studies , zirconia post and core should be clinically recommended .
this needs future investigations . considering the optical properties of zrc and the thickness of zirconia sub - structures , which is approximately 0.5 mm in a normal case
, light transmission through the zirconia structure can be expected in zirconia - based restorations .
the present study evaluated the effects of three core materials in this respect . on the other hand ,
the zirconia core overlaying materials such as porcelain veneers and glaze may influence the final color of zirconia - based restorations , which were not assessed in this study .
therefore , evaluation of the effect of other factors in this respect is recommended in future studies .
the present study had some limitations such as using a specific brand of zrc , an uncolored zrc , and a specific brand of composite resin .
within the limitations of this study , it was concluded that the tested zrc could not thoroughly mask the three core materials of nca , npga , and zrc .
 objectives : masking ability of a restorative material plays a role in hiding colored substructures ; however , the masking ability of zirconia ceramic ( zrc ) has not yet been clearly understood in zirconia - based restorations . 
 this study evaluated the effect of three different core materials on masking ability of a zrc.materials and methods : ten zirconia disc samples , 0.5 mm in thickness and 10 mm in diameter , were fabricated . 
 a white ( w ) substrate ( control ) and three substrates of nickel - chromium alloy ( nca ) , non - precious gold alloy ( npga ) , and zrc were prepared . 
 the zirconia discs were placed on the four types of substrates for spectrophotometry . 
 the l * , a * , and b * values of the specimens were measured by a spectrophotometer and color change ( e ) values were calculated to determine color differences between the test and control groups and were then compared with the perceptual threshold . 
 randomized block anova and bonferroni test analyzed the data . 
 a significance level of 0.05 was considered.results:the mean and standard deviation values of e for nca , npga , and zrc groups were 10.262.43 , 9.451.74 , and 6.701.91 units , respectively . 
 significant differences were found in the e values between zrc and the other two experimental groups ( nca and npga ; p<0.0001 and p=0.001 , respectively ) . the e values for the groups were more than the predetermined perceptual threshold.conclusions:within the limitations of this study , it was concluded that the tested zrc could not well mask the examined core materials . 
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
CONCLUSION