with the increase in the demand for possessing a beautiful smile and white teeth in recent years , management of discolored teeth has high importance in aesthetic dentistry . depending on the severity of discoloration ,
there are several treatment options including vital and nonvital bleaching , micro abrasion , composite and porcelain veneers , porcelain crowns and sometimes a combination of them .
all - ceramic restorations are more translucent and thus have more aesthetic properties than restorations with metal substrates and can be used in aesthetic areas properly .
it has been proven that porcelain veneers are very efficient for treating discolored teeth , and they last for a long time if they bond properly to the tooth structure .
although limiting the preparation to enamel leads to more efficient bonding , the porcelain restoration should be also thick enough to mask the discoloration .
however , in treating a deeply discolored tooth , a full coverage crown might be the ultimate option .
therefore , determining the minimum thickness of a porcelain restoration required for masking heavily discolored teeth can be very useful in clinical treatments .
there are several factors that determine the final aesthetic properties of an all - ceramic restoration in vivo : color of the ceramic , thickness and the combination of ceramic layers ( such as core and veneer with different shade and opacity ) , the thickness and the color of the luting agent and the color of underlying tooth structure .
the ability of an all - ceramic restoration for masking a tooth with severe discoloration can be determined by measuring the color difference ( e*ab ) when the restoration is placed on two different backgrounds : a dark background and a background fabricated with the same material as the tested restoration but with enough thickness to be completely opaque .
the masking ability can be determined using the following formula [ eq . 1 ] .
spectrophotometer is used to determine international commission on illumination ( french commission internationale de lclairage , hence its cie initialism ) l*a*b * ( cielab ) color coordinates by spectral reflectance measurements .
l * , a * and b * are representatives of lightness , greenness - redness and yellowness - blueness respectively . when there is no color difference ( e*ab = 0 ) , the masking ability of the system is perfect .
however , it has been shown that the visually acceptable color difference , for dental applications , is when e*ab is 3.3 .
heat - pressed glass - ceramic lithium disilicate - reinforced ceramics ( ips e.max press , ivoclarvivadent , schaan , liechtenstein ) is a lithium silicate glass ceramic with proper aesthetic features and strength which can be used as extremely thin anterior veneers .
concluded that thin porcelain veneers ( ips e.max press , a1 , 0.6 mm thick , ivoclarvivadent ) can not mask the underlying tooth color even when different shades of resin cements are used .
according to the results of a study by zhou et al . , high opaque ( ho ) series of ips e.max press disks with 0.6 mm , 0.8 mm and 1 mm thicknesses could mask the metal substrate ( e*ab < 1.5 ) shono and al nahedh demonstrated that 1.5 mm thickness of ips e.max ( a2-shade ) was not able to completely mask the black background .
however , the influence of severe discolored tooth on a multilayer ( core and veneer ) ips e.max restoration has not been established previously . therefore , we investigated this aspect of influence on ips e.max press by measuring its l * , a * and b * values .
the purpose of this in vitro study was to determine the minimum thickness of a multilayer all - ceramic restoration ( ips e.max press ) required for masking severe tooth discoloration ( c4-shade ) including a ho core for masking the underlying discoloration and an a1 shade veneering layer to provide some translucency simulating tooth enamel .
in addition , panavia f 2.0 light ( kurrary , kurashiki , okayama , japan ) resin cement was used to enhance the masking ability .
the null hypothesis was that all of the selected thicknesses for the ceramic restoration in corporation with the selected resin cement would mask the c4-shade background .
in this in vitro study , a total of 24 disk - shaped multilayer specimens were prepared from ips e.max press ( ivoclar vivadent , schaan , liechtenstein ) , which is a lithium disilicate glass ceramic in form of ingots for the heat - press technique .
different thicknesses of disks were fabricated as recommended by the manufacturer using lost wax and heat pressed techniques . for ips e.max press ceramic , the baseline disk thickness was 0.4 mm of core and of 0.4 mm of veneer ( group 1 ) .
additional specimens were made with increasing core / veneer thickness ( mm ) of 0.5/0.5 ( group 2 ) , 0.6/0.6 ( group 3 ) and 0.8/0.7 ( group 4 ) according to the manufacturer 's instruction . according to a similar study of color analysis on ips e.max ceramic , sample size
ho1 shade was selected to fabricate the ceramic core layer according to the manufacturer 's recommendation for masking severe tooth discoloration . to obtain the desired thicknesses ,
cores wax patterns were fabricated by pixcera machine ( perfactory , gladbeck , germany ) .
sets containing three wax patterns were invested in an investment ring with a phosphate - bonded investment ( ivoclar vivadent , schaan , liechtenstein ) after attachment of a 3 mm - diameter sprue to each of the patterns .
the rings were bench set for 60 min and placed into a burn out furnace ( ivoclar vivadent , schaan , liechtenstein ) for 120 min .
the specimens were then heat - pressed in an ep600 furnace ( ivoclar vivadent , schaan , liechtenstein ) , air - cooled , divested by blasting with 80  glass beads at 4 bar pressure , and ultrasonically treated in an acidic cleaning liquid ( invex , ivoclar vivadent , schaan , liechtenstein ) .
the thickness were measured using a digital caliper with 0.01 mm resolution ( mitutoyo digimatic , kawasaki , japan ) and were adjusted if needed with 350 , 600 and 1500 grit silicon carbide paper ( matador wasserfast , germany ) under running water . according to the manufacturer 's instruction
a thin layer of ips e.max deep dentin should be applied on ho cores before appling a more translucent veneering layer .
we applied a 0.2 mm thickness of deep dentin ( a1 shade ) using a digital caliper . in the next step
, the core disks were veneered with a1 shade of ips e.max ceram layering material ( mixture of 50% t and 50% ti shades , ivoclar vivadent , schaan , liechtenstein ) . to obtain a uniform thickness ,
the core disks were inserted into custom made brass molds with 13 mm internal diameter and 1 , 1.2 , 1.4 and 1.7 mm thicknesses to the desired depth .
these molds were fabricated 0.2 mm thicker than the desired final thicknesses to compensate for the veneering layers shrinkage .
the veneering porcelain was mixed with build - up liquids and introduced to the molds with hand vibration and condensation .
the disks were ground on the veneer side using the same polishing apparatus with 350 , 600 and 1500 grit silicon carbide paper under running water to adjust to the designated total thicknesses and an auto - glazing process was performed at 730c .
later , the disks were immersed in ips e.max press invex liquid for 20 min ( < 1% hydrofluoric acid ) and cleaned in an ultrasonic cleaner ( dentsply neytech , ca , usa ) for 10 min and then cleaned with airborne - particle abrasion using 100  al2o3 powder at 2 bar pressure ( bego , zirodent dentalhandel gbr , .
finally , etching of the disks was done using ips ceramic etching gel ( 4.5% hydrofluoric acid ) for 20 s. afterwards , all specimens were cleaned in an ultrasonic bath ( ultrasound vita - sonic ii , vita zahnfabrik , germany ) for 5 min and dried .
two backgrounds ( c4-shade body porcelain [ c4 ] and an opaque background from the selected ips e.max ceramic itself , and opaque ceramic [ oc ] ) were used to mimic a discolored or stained natural tooth structure and to determine the masking ability .
for oc background , a 4 mm - thick specimen ( core and veneer = 2.2 and 1.8 ) was fabricated using the waxing machine mentioned before for the core ( ho1 ) and a split brass ring mold ( with 4.2 mm thickness ) for the veneerig layer ( mixture of 50% t and 50% ti shades ) .
deep dentin ( a1 : 0.2 mm ) was applied between the core and veneering layer .
the c4 plate ( vita vmk68 , vita zahnfabrik , badsackingen , germany ) was fabricated with a 4.2 mm - thick mold and its inherent cielab values were measured .
the thickness sufficient to mask a discolored tooth structure ( critical thickness ) was determined by calculating the e1*a*b of specimens between the c4 and oc backings .
we selected the panavia f 2.0 ( kurrary , kurashiki , okayama , japan ) light cure resin cement to benefit from its partial masking ability .
since the luting cement was the same for all ceramic specimen in our study , we placed the cement layer on the two backgrounds instead of applying it to the 24 ceramic specimens . for applying the cement layer on each background
, a disc with 13 mm in diameter was obtained from a plastic sheet which had 0.1 mm thickness , placed on the background and silicone mold was prepared .
four gaps were made in the silicon mold for the cement excess . inside the space ,
first , a plastic sheet and the glass slab were put on the uncured resin cement and pressed with finger and cured by quartz - tungsten - halogen light curing device for 40 s ( optilux 501 , demetron kerr , orange , ca , usa ) [ figure 1 ] .
the color measurements were performed using a gretag macbeth coloreye 7000a spectrophotometer ( color eye 7000 a , model c6 ; gretag macbeth , new windsor , ny , usa ) .
this spectrophotometer with an integrating sphere d/8 geometry has two measuring modes ; specular component included and specular components excluded ( sce ) . in the present study the specular excluded ( sce ) configuration was applied to compensate for errors caused by surface glaze . before each measurement
internationale de leclairage colorimetry color is quantified using the parameters of lightness ( l * ) and chromaticity along the red - green ( a * ) and yellow - blue ( b * ) axes .
the difference between two colors ( e ) is determined by measuring cielab values by a spectrophotometer .
cielab coordinates provide a numerical description of the color 's position in a three - dimensional color space .
the a * color usually coordinate ranges from 90 to 70 and represents the greenness on the negative axis and redness on the positive .
the b * color coordinate ranges almost from 80 to 100 and represents yellowness ( positive b * ) and blueness ( negative b * ) .
e*ab represents the numerical distance between l*a * b * coordinates of 2 colors using the eq .
the e*ab value ( difference between c4   and oc ) was evaluated for each thickness .
1 . a smaller e indicates that the specimen is less sensitive to ( as better able to mask ) the c4-shade background color .
critical thickness is the minimum ceramic thickness sufficient for masking the c4-background that is determined through the clinically acceptable e range ( e*ab  3.3 ) . determining the minimum thickness of ceramic for masking c4-shade background ( critical thickness ) is a cut - off point , which does not need any statistical analysis .
for the four different thicknesses specimen groups , one - way anova and post - hoc tests of specimen ( tukey hsd ) were conducted , and p  0.05 was set as the level of significance .
differences between means at p  0.05 was considered statistically significant . to make a statistical analysis , spss 21 computer program ( spss inc . , chicago , il , usa ) was used .
in this in vitro study , a total of 24 disk - shaped multilayer specimens were prepared from ips e.max press ( ivoclar vivadent , schaan , liechtenstein ) , which is a lithium disilicate glass ceramic in form of ingots for the heat - press technique .
different thicknesses of disks were fabricated as recommended by the manufacturer using lost wax and heat pressed techniques . for ips e.max press ceramic , the baseline disk thickness was 0.4 mm of core and of 0.4 mm of veneer ( group 1 ) .
additional specimens were made with increasing core / veneer thickness ( mm ) of 0.5/0.5 ( group 2 ) , 0.6/0.6 ( group 3 ) and 0.8/0.7 ( group 4 ) according to the manufacturer 's instruction . according to a similar study of color analysis on ips e.max ceramic , sample size
ho1 shade was selected to fabricate the ceramic core layer according to the manufacturer 's recommendation for masking severe tooth discoloration . to obtain the desired thicknesses ,
cores wax patterns were fabricated by pixcera machine ( perfactory , gladbeck , germany ) .
sets containing three wax patterns were invested in an investment ring with a phosphate - bonded investment ( ivoclar vivadent , schaan , liechtenstein ) after attachment of a 3 mm - diameter sprue to each of the patterns .
the rings were bench set for 60 min and placed into a burn out furnace ( ivoclar vivadent , schaan , liechtenstein ) for 120 min .
the specimens were then heat - pressed in an ep600 furnace ( ivoclar vivadent , schaan , liechtenstein ) , air - cooled , divested by blasting with 80  glass beads at 4 bar pressure , and ultrasonically treated in an acidic cleaning liquid ( invex , ivoclar vivadent , schaan , liechtenstein ) .
the thickness were measured using a digital caliper with 0.01 mm resolution ( mitutoyo digimatic , kawasaki , japan ) and were adjusted if needed with 350 , 600 and 1500 grit silicon carbide paper ( matador wasserfast , germany ) under running water . according to the manufacturer 's instruction
a thin layer of ips e.max deep dentin should be applied on ho cores before appling a more translucent veneering layer .
we applied a 0.2 mm thickness of deep dentin ( a1 shade ) using a digital caliper . in the next step
, the core disks were veneered with a1 shade of ips e.max ceram layering material ( mixture of 50% t and 50% ti shades , ivoclar vivadent , schaan , liechtenstein ) . to obtain a uniform thickness ,
the core disks were inserted into custom made brass molds with 13 mm internal diameter and 1 , 1.2 , 1.4 and 1.7 mm thicknesses to the desired depth .
these molds were fabricated 0.2 mm thicker than the desired final thicknesses to compensate for the veneering layers shrinkage .
the veneering porcelain was mixed with build - up liquids and introduced to the molds with hand vibration and condensation .
the disks were ground on the veneer side using the same polishing apparatus with 350 , 600 and 1500 grit silicon carbide paper under running water to adjust to the designated total thicknesses and an auto - glazing process was performed at 730c .
later , the disks were immersed in ips e.max press invex liquid for 20 min ( < 1% hydrofluoric acid ) and cleaned in an ultrasonic cleaner ( dentsply neytech , ca , usa ) for 10 min and then cleaned with airborne - particle abrasion using 100  al2o3 powder at 2 bar pressure ( bego , zirodent dentalhandel gbr , .
finally , etching of the disks was done using ips ceramic etching gel ( 4.5% hydrofluoric acid ) for 20 s. afterwards , all specimens were cleaned in an ultrasonic bath ( ultrasound vita - sonic ii , vita zahnfabrik , germany ) for 5 min and dried .
two backgrounds ( c4-shade body porcelain [ c4 ] and an opaque background from the selected ips e.max ceramic itself , and opaque ceramic [ oc ] ) were used to mimic a discolored or stained natural tooth structure and to determine the masking ability . for oc background ,
a 4 mm - thick specimen ( core and veneer = 2.2 and 1.8 ) was fabricated using the waxing machine mentioned before for the core ( ho1 ) and a split brass ring mold ( with 4.2 mm thickness ) for the veneerig layer ( mixture of 50% t and 50% ti shades ) .
deep dentin ( a1 : 0.2 mm ) was applied between the core and veneering layer .
the c4 plate ( vita vmk68 , vita zahnfabrik , badsackingen , germany ) was fabricated with a 4.2 mm - thick mold and its inherent cielab values were measured .
the thickness sufficient to mask a discolored tooth structure ( critical thickness ) was determined by calculating the e1*a*b of specimens between the c4 and oc backings .
we selected the panavia f 2.0 ( kurrary , kurashiki , okayama , japan ) light cure resin cement to benefit from its partial masking ability .
since the luting cement was the same for all ceramic specimen in our study , we placed the cement layer on the two backgrounds instead of applying it to the 24 ceramic specimens . for applying the cement layer on each background
, a disc with 13 mm in diameter was obtained from a plastic sheet which had 0.1 mm thickness , placed on the background and silicone mold was prepared .
four gaps were made in the silicon mold for the cement excess . inside the space ,
first , a plastic sheet and the glass slab were put on the uncured resin cement and pressed with finger and cured by quartz - tungsten - halogen light curing device for 40 s ( optilux 501 , demetron kerr , orange , ca , usa ) [ figure 1 ] .
the color measurements were performed using a gretag macbeth coloreye 7000a spectrophotometer ( color eye 7000 a , model c6 ; gretag macbeth , new windsor , ny , usa ) .
this spectrophotometer with an integrating sphere d/8 geometry has two measuring modes ; specular component included and specular components excluded ( sce ) . in the present study the specular excluded ( sce ) configuration was applied to compensate for errors caused by surface glaze . before each measurement
internationale de leclairage colorimetry color is quantified using the parameters of lightness ( l * ) and chromaticity along the red - green ( a * ) and yellow - blue ( b * ) axes .
the difference between two colors ( e ) is determined by measuring cielab values by a spectrophotometer .
cielab coordinates provide a numerical description of the color 's position in a three - dimensional color space .
the a * color usually coordinate ranges from 90 to 70 and represents the greenness on the negative axis and redness on the positive .
the b * color coordinate ranges almost from 80 to 100 and represents yellowness ( positive b * ) and blueness ( negative b * ) .
e*ab represents the numerical distance between l*a * b * coordinates of 2 colors using the eq .
the e*ab value ( difference between c4   and oc ) was evaluated for each thickness .
1 . a smaller e indicates that the specimen is less sensitive to ( as better able to mask ) the c4-shade background color .
critical thickness is the minimum ceramic thickness sufficient for masking the c4-background that is determined through the clinically acceptable e range ( e*ab  3.3 ) . determining the minimum thickness of ceramic for masking c4-shade background ( critical thickness ) is a cut - off point , which does not need any statistical analysis .
for the four different thicknesses specimen groups , one - way anova and post - hoc tests of specimen ( tukey hsd ) were conducted , and p  0.05 was set as the level of significance .
differences between means at p  0.05 was considered statistically significant . to make a statistical analysis , spss 21 computer program ( spss inc . , chicago , il , usa ) was used .
according to the spectrophotometric measurements , l * , a * and b * values of the backgrounds are presented in table 1 .
the mean cielab color values and e values of ips e.max ceramic specimens placed on both backgrounds are shown in tables 2 and 3 .
chromatic values of ceramic backgrounds chromatic values of ceramic specimen under different backgrounds ( mean  sd ) e*ab of ceramic specimen over different backgrounds ( mean  sd ) e*ab of all groups were within the range of the clinically acceptable color difference ( e 3.3 ) , thus all the groups could mask the c4 background .
a trend was showing the results as by increasing the thickness , e*ab was decreased .
the mean e*ab between different thicknesses was statistically significant ( p < 0.05 ) only between groups 1 and 4 ( mean difference of 0.71 ) and between 2 and 4 ( mean difference of 0.59 ) .
according to the results of our study , the minimum thickness of a multilayer porcelain restoration ( ips e.max press ) required for masking severe tooth discoloration was 0.8 mm .
thus , our null hypothesis that all the specimen groups would mask the c4-shade background was supported .
management of tooth discoloration caused from intrinsic or extrinsic factors is one of the major challenges any dentist may face .
depending on the etiology and severity of the tooth discoloration , treatment options can range from a simple scaling and polishing of the teeth and different bleaching procedures to restorative treatments such as veneers or even crowns . in treating sever tooth discolorations , restoring an anterior tooth by an all - ceramic restoration with the advantages such as excellent aesthetic properties , biocompatibility and wear resistance can be an appropriate option .
but , there is a question that how much tooth reduction is necessary to mask the severe discoloration ?
although a thin porcelain veneer of an opaque ceramic can mask the underlying discolored tooth , it leads to a nonvital appearance .
therefore , in our study we tried to find the minimum thickness of a multilayer ceramic restoration with an opaque core and a translucent veneer to not only mask the tooth discoloration , but also to have a vital appearance by simulating the translucency of the natural tooth enamel .
although increasing the thickness of a ceramic restoration improves its masking ability , the increased amount of tooth reduction thereby can jeopardize pulpal health .
on the other hand , an efficient bonding of a ceramic restoration can be achieved when the tooth preparation is limited to the enamel .
in addition , increasing the opacity of the ceramic restoration adversely affects its aesthetic properties .
therefore , using a multilayer ceramic restoration including an opaque core for masking the underlying discoloration and also a veneering layer to give some translucency in combination with an opaque luting cement would be beneficial . in the present study
, we used a c4-shade dental porcelain to simulate a severely discolored tooth and evaluated the ability of different thicknesses of ips e.max ceramic specimens for masking this discoloration by measuring their cielab values .
besides color differences , the masking ability of ceramic materials can be evaluated with a spectrophotometric instrument in terms of the opacity or contrast ratio ( cr ) .
the ( cr = yb / yw ) is defined as the ratio of illuminance ( y ) of the test material when it is placed over a black background ( yb ) to the illuminance of the same material when it is placed over a white background ( yw ) . according to some studies , when the numerical distance between l*a * b * coordinates of 2 colors ( e )
is < 1 unit , it means that the colors are match and when it is between 1 and 2 , the difference is frequently detected by observers and e of higher than 2 is detectable by all observers .
however , e of below 3.7 in oral environment is unnoticeable because of the uncontrolled clinical condition .
e values of < 1 unit were regarded as not appreciable by the human eye ; e values > 1 and < 3.3 units were considered appreciable by skilled operators , but clinically acceptable ; e values > 3.3 were considered perceivable by untrained observers ( e.g. , patients ) , and for that reason were regarded as not acceptable . in this study ,
all groups had e*ab of between 1 and 2 . according to some other studies ,
differences in e*ab lower than 1.1 can not be detected by the human eye , a e*ab between 1.1 and 3.3 can be detected but is still considered clinically acceptable , while a e*ab higher than 3.3 can be detected and is by an aesthetic point of view considered as clinically not acceptable .
we also considered e*ab < 3.3 as a clinically acceptable color difference in our study .
the overall optical behavior and aesthetic properties of an all - ceramic restoration are determined by the underlying tooth structure color , the thickness of the ceramic layers , and the color of the cement .
vichi et al . investigated the masking ability of ceramic restorations of various thicknesses ( 1.0 , 1.5 , and 2.0 mm ) of a leucite - reinforced ceramic material ( ips empress ; ivoclar vivadent , schaan , liechtenstein ) over different opaque posts and reported that full masking or acceptable e was achieved only with the 2 mm - thick ceramic material .
results of a study by shimada et al . revealed that there were no significant differences in e*ab value at 2.0 mm thickness of ips empress2 ingot-100 under different backgrounds ( a composite build - up material , a gold alloy and a silver palladium alloy ) .
suggested that procera and empress 2 had significantly higher cr and masking abilities when compared to vitadur alpha .
however , when the discoloration is too intense , the application of these two materials may still be limited . in this study , we investigated the masking ability of ips e.max press . by the tremendous advances in the mechanical and optical properties and fabrication methods of ceramic materials , heat - pressed glass - ceramic lithium disilicate - reinforced ceramics ( ips e.max press )
ips e.max press with the compressive strength of 350 - 450 mpa and fracture toughness of approximately 3 times more that of the lucite glass ceramic also has the ability of bonding to tooth structure .
its fabrication technique ( lost wax ) is more practical than layering technique and leads to an excellent adaption of the restoration .
it is the optical compatibility between the glassy matrix and the crystalline phase which makes the glass - ceramic lithium disilicate - reinforced ceramics high translucent aesthetic ceramics . including various shades with different chroma and opacities gives the dentist the opportunity of achieving the desired color even in treating severe discolored teeth .
an investigation on masking ability of ips e.max computer - aided design / computer - aided manufacturing low translucent in a group with a dark - colored abutment tooth revealed that 1.0 mm of this restorative material cemented using either translucent cement or opaque cement and 1.5 mm cemented with translucent cement had e values higher than the clinically unacceptable range ( e > 3.7 ) .
but we concluded that by using a restoration from ips e.max press with only 0.8 mm thickness in combination with panavia resin cement on c4 shade abutment , e*ab would be in clinically acceptable range .
zhou et al . concluded that ips e.max press ho series ceramic specimen thickness of 0.4 mm could not guarantee completely masking the metal substrate background and produced clinically unacceptable color match . while the thicknesses of 0.6 mm , 0.8 mm and 1.0 mm could significantly mask the color of metal substrate disks ( e < 1.5 ) . in our study , groups 3 ( 1.2 mm ) and 4 ( 1.5 mm ) had e*ab of < 1.5 .
studies in this field , however , did not address in particular the minimum thickness of multilayer ips e.max press required masking a discolored tooth .
the purpose of this in vitro study was to determine the minimum thickness of all - ceramic restoration required to mask a c4-shade background while using panavia f2 resin cement to enhance the masking ability . to achieve the best aesthetic results in masking a severe discoloration , our ceramic specimen included two ceramic layers with different properties ; a ho core layer to mask the underlying discoloration and a translucent veneering layer to replicate the natural tooth vitality appearance .
our results showed that the minimum thickness of the porcelain required for masking severe tooth discoloration was 0.8 mm . by limiting the tooth preparation to only 0.8 mm ( in the enamel )
, we can get a more predictable bonding of the ceramic restoration to the tooth structure . in our study ,
e*ab of all groups were lower than 3.3 or 3.7 which are clinically acceptable color difference . even considering the perceptible difference ( e > 2 ) , still all groups had the ability of masking the discolored background .
the minimum thickness of a multilayer porcelain restoration ( ips e.max press ) required for masking severe tooth discoloration was 0.8 mm including a 0.4 mm core and 0.4 mm veneer .
the authors of this manuscript declare that they have no conflicts of interest , real or perceived , financial or non - financial in this article .
the authors of this manuscript declare that they have no conflicts of interest , real or perceived , financial or non - financial in this article .
 background : although studies have shown that porcelain veneers are very efficient for treating discolored teeth , they did not address in particular the minimum thickness of a multilayer ips e.max press ( ivoclarvivadent , schaan , liechtenstein ) restoration required to mask discolored tooth . 
 the aim of this study was to determine the minimum thickness of a multilayer porcelain restoration required for masking severe tooth discoloration.materials and methods : a total of 24 disk - shaped multilayer specimens were prepared from ips e.max press with the diameter of 13 mm and four different thicknesses ( core / veneer : 0.4/0.4 mm , 0.5/0.5 mm , 0.6/0.6 mm and 0.8/0.7 mm ) . 
 two backgrounds , c4-shade body porcelain and an opaque background from the selected ips e.max ceramic itself were fabricated to mimic a discolored or stained natural tooth structure and to determine the masking ability . after applying the resin cement layer ( panavia f2.0 ) with 0.01 mm thickness on each background , all specimens were measured on both background using a spectrophotometer and values of l * , a * and b * were calculated to determine the color differences ( e*ab ) . 
 one - way anova and post - hoc tests of specimen average one - to - one comparison ( tukey hsd ) were conducted and p  0.05 was set as the level of significance.results:e*ab of all groups were within the range of the clinically acceptable color difference ( e3.3 ) , thus all the groups could mask the c4 background even group 1 with only 0.8 mm thickness . 
 a trend was shown in the results as by increasing the thickness , e*ab is was decreased . 
 the mean e*1*a*b between different thicknesses were statistically significant ( p < 0.05 ) only between group 4 with groups 1 and 2 , respectively.conclusion:within the limitations of this study , all studied thicknesses could mask the c4 background . 
 however , the minimum thickness of a multilayer porcelain restoration ( ips e.max press ) required for masking severe tooth discoloration was 0.8 mm . 
INTRODUCTION
MATERIALS AND METHODS
Fabrication of ceramic specimens
Fabrication of backgrounds
Spectrophotometric analysis
RESULTS
DISCUSSION
CONCLUSION
Financial support and sponsorship
Conflicts of interest