ceramic and composite resin , esthetic dental materials are continuously developing to meet functional and esthetic demand .
indirect restorations require appropriate cement for long - term use in changeable oral conditions and also to meet patients esthetic expectations . in success of indirect restorations ,
it is important that indirect restorations are firmly attached to the tooth structure with resin - based cement .
several different types of light curing unit ( lcu ) , from conventional quartz - tungsten - halogen ( qth ) lamps to argon laser , have been employed .
qth lamps emit light when electrical energy heats a small tungsten filament to high temperatures .
qth - curing lights work at wavelengths of 400 to 500 nm with output ranging from 400 to 800 mw / cm .
light - emitting diode ( led ) or solid - state led , instead of the hot filaments used in halogen bulbs , leds use junctions of doped semiconductors for generating light .
recently ,  polywave  led units ( with two or more spectral peaks ) have been introduced using two or more different led colors , meaning that their spectral output ranges from blue ( 460 nm ) to violet wavelengths ( 410 nm ) of light .
these lights can polymerize composite resins containing both conventional and alternative photoinitiators . since most manufacturers of direct resin - based restorative materials do not provide details about the photoinitiators contained in their products ,
narrow blue led light spectra emission is ideal for photopolymerization of camphorquinone containing materials , but this may not be such efficient when other photoinitiators , such as 1-phenyl-1,2-propanedione , are present in very light composite shades ( high color value ) used for repair of bleached teeth .
this situation was addressed by manufacturing dual - wave ( or  polywave  ) led lcus , emitting in the violet ( 410 nm ) and blue ( 470 nm ) component of the light spectrum . the third generation , polywave leds are used in these led lcus .
while many publications have discussed the physical properties of led light - cured materials , few have investigated the color stability of resin - based restorative materials when cured with qth and led lights .
monowave and polywave led lcus and qth lcus have not previously been compared in terms of efficiency in curing resin cement under indirect restorative materials .
shade matching , whether by visual or instrumental methods , requires an understanding of color harmony and tolerance . in other words ,
the e value of 2.5 exhibited a borderline value recognizable to all people in a color test .
color changes are considered visually perceptible when e > 1 and clinically acceptable when e < 3.3 .
the cie l*a*b * - based color difference formula is most widely used . cie l*a*b * color appearance attributes are frequently used to calculate cie l*a*b * color difference ( e ) as equations .
color differences (  e ) were calculated using the cielab35 color notation system namely , l * ( lightness , ranging from 0 to 100 with higher numbers being brighter ) , a * ( green - red direction ) , and b * ( blue is the yellow direction ) .
cielab parameters ( l * , a * , and b * ) of each specimen were recorded at initial ( i ) and at final ( f ) .
the purpose of this study was , therefore , to evaluate the effect of color changes in a resin cement material polymerized under different restorations using different light polymerizing units .
the hypothesis of this study would be different curing systems and different interface materials effects the resin cement color stability at different time interval .
conventional halogen ( qth ) , polywave and monowave led lcus , and interface materials ( polystyrene strip , ceramic , and composite resin ) were used .
a list of the curing lights , including specifications and manufacturers , is given in table 1 ( kuraray , japan , lot : 0015 aa ) resin cement was used for all test groups . curing regimes and curing conditions
the protocol involved 18 groups ( n = 5 ) ; three interface materials and three different curing systems at two - time intervals .
resin cement was firmly pressed under a 5 kgf load for 3 min in a teflon cylindrical mold under a glass slide covered by a polystyrene strip ( kerrhawe stopstrip , kerrhawe sa , bioggio , switzerland , lot : 70501264 ) to produce a uniform thickness .
resin cements were polymerized on interfaces prefabricated from ceramic ( a2 vita porcelain , sckingen , germany ) and composite resin ( a2 esthet x hd , dentsply , caulk , milford , de , lot no : 090119 ) for simulated ceramic or composite restoration .
the resin cement was exposed to light through the upper glass slide in contact with a curing 9-mm light tip for 20 or 40 s in the case of qth units , 20 or 40 s for monowave led units , and 3 or 6 s for polywave led units [ table 1 ] .
the output of the curing light was checked with a radiometer ( curing radiometer , model 100 , kerr corp . ,
no polishing techniques were used in order to avoid modification of the surfaces that might have influenced the results .
the color of each specimen was measured three times ; before polymerization and twice after polymerization ( 20/40 s for qth and monowave led and 3/6 s for polywave led ) using a colorimeter ( shade eye nnc , shofu , japan ) .
the measurements were performed , according to the cie l*a*b * system , and mean l * , a * , and b * values for each material were calculated . the total color change ( e ) obtained was calculated for each specimen using the equation : where l * , a * , and b * are differences in the respective values before and after polymerization .
data obtained were analyzed using three - way analysis of variance ( anova ) followed by duncan test significant difference tests for comparisons among groups at a 0.05 level of significance .
conventional halogen ( qth ) , polywave and monowave led lcus , and interface materials ( polystyrene strip , ceramic , and composite resin ) were used .
a list of the curing lights , including specifications and manufacturers , is given in table 1 ( kuraray , japan , lot : 0015 aa ) resin cement was used for all test groups . curing regimes and curing conditions
the protocol involved 18 groups ( n = 5 ) ; three interface materials and three different curing systems at two - time intervals .
resin cement was firmly pressed under a 5 kgf load for 3 min in a teflon cylindrical mold under a glass slide covered by a polystyrene strip ( kerrhawe stopstrip , kerrhawe sa , bioggio , switzerland , lot : 70501264 ) to produce a uniform thickness .
resin cements were polymerized on interfaces prefabricated from ceramic ( a2 vita porcelain , sckingen , germany ) and composite resin ( a2 esthet x hd , dentsply , caulk , milford , de , lot no : 090119 ) for simulated ceramic or composite restoration .
the resin cement was exposed to light through the upper glass slide in contact with a curing 9-mm light tip for 20 or 40 s in the case of qth units , 20 or 40 s for monowave led units , and 3 or 6 s for polywave led units [ table 1 ] .
the output of the curing light was checked with a radiometer ( curing radiometer , model 100 , kerr corp . ,
no polishing techniques were used in order to avoid modification of the surfaces that might have influenced the results .
the color of each specimen was measured three times ; before polymerization and twice after polymerization ( 20/40 s for qth and monowave led and 3/6 s for polywave led ) using a colorimeter ( shade eye nnc , shofu , japan ) .
the measurements were performed , according to the cie l*a*b * system , and mean l * , a * , and b * values for each material were calculated . the total color change ( e )
obtained was calculated for each specimen using the equation : where l * , a * , and b * are differences in the respective values before and after polymerization .
data obtained were analyzed using three - way analysis of variance ( anova ) followed by duncan test significant difference tests for comparisons among groups at a 0.05 level of significance .
resin cement colorimetric values ( l*a*b * ) of the resin cement polymerized under ceramic and composite discs with three light polymerizing units before and after polymerization are given in table 2 .
color changes ( e ) of all test groups are presented table 3 and figure 1 .
l * , a * , b values ( mean  sd ) of the test groups ( n=5 ) e values and sd of the test groups ( n = 5 ) color changes ( e ) of all test groups are presented three - way anova revealed significance for interface materials and curing units in relation to e ( p < 0.05 ) .
the interaction between polymerizing units , material and time was not significant ( p > 0.05 ) [ table 4 ] .
results of three - way anova both interface materials , ceramic and composite resin are induced less color change than the control group ( p < 0.05 ) .
monowave led exhibited significantly higher color changes than the other materials ( p < 0.05 ) ( e 2.94 ) .
composite specimens with qth induced significantly less color changes ( [ p < 0.05 ] [ e 0.87  0.41 ] ) [ figure 1 ] .
this study investigated the color of indirect composite or ceramic restoration , after cementation with resin cement using differently lcus .
the color stability of both indirect restorations and luting materials is important for the esthetics of laminate veneers , all - ceramic crowns , and indirect composite restorations .
one of the most important benefits of ceramic crowns is the semi - translucency of the material that allows light transmission .
this optical property improves the esthetics of ceramic crowns as compared to metal - ceramic crowns that do not allow light transmission through the metal .
however , the translucency of esthetic indirect restorations is very important in terms of the complex color matching process .
the color of the tooth and the luting cement can affect the appearance of indirect restorations .
several studies have reported the importance of color to the clinical appearance of an esthetic crown .
led lcus have become the gold standard for photopolymerization of resin - based dental materials .
since contemporary led lights provide superior irradiance , expanded lifetime , little light output degradation over time to qth light , led lights are expected to optimally polymerize resin materials as or even more effectively than qth . in the current study ,
color changes in specimens polymerized with monowave led were statistically greater than those in halogen - based polymerizing units and polywave led lcus ( p < 0.05 ) .
polywave led devices portrayed as having maximum theoretical polymerization efficacy also have characteristics that need to be developed . in some of these  polywave  led units , the spectral emission is not uniformly distributed across the light tip , further compounding the effects of beam inhomogeneity .
this may explain the color changes in the specimens polymerized with polywave led lcus used in this study being greater than the color change values in specimens polymerized with monowave led lcus . in the present study , significant difference was showed between interface materials prepared for the purpose of representing indirect restorations .
the greatest color change was obtained in the control group since polymerization was more effective ( p < 0.05 ) .
ceramic and composite interface materials were seen to be able to cause a decrease in polymerization efficacy .
inadequate polymerization or cement type factors that may cause the color changes followed by polymerization .
clearfil sa cement is a dual cure cement and polymerized in two stages with physically and chemically .
such reaction may occur a higher monomer conversion which allows the polymerization of the deeper layers of the resin
. however , the thickness and light transmittance of the indirect restoration , which applied on the resin cement , should be able to allow sufficient polymerization of the resin cement . in our study ,
similarly , the ceramic disc used in our study is more translucent than composite , and it was caused a higher polymerization by passing more light from composite block .
watanabe et al . has shown that high - intensity led unit is more effective than qth units during polymerization of dual cure cement trough ceramic material .
has reported that led units emit more homogenous light and thus yield more uniform distribution of surface hardness ( knoop hardness number ) of a resin composite than halogen units . in our study , monowave led exhibited significantly higher color changes .
the color appearance of a layered ceramic disk specimen is strongly influenced by the core and veneer thicknesses and also by their interaction between them .
in contrast , a 1.0-mm thick ips empress 1 ceramic material was not affected by the shades of substrate and luting composite cement tested .
the effects of two different cement shades ( vita a1 and a3 ) and water storage on the leucite - reinforced ceramic color over time showed no changes in the experimental design may explain the different results from different studies , such as the contents of materials , color of the test specimens , and the color - measuring instruments .
it is also important to realize the impossibility of establishing exact comparability between in vitro and in vivo tests .
considering the methodology applied and the limitations of this in vitro study , it can be concluded that different curing units and interface materials have a significant effect on the final color of resin cement .
more studies are now needed to evaluate the efficiency of polymerization of mono / polywave led lcus and to evaluate the effect of different interface materials on color stability of resin cement .
 aim : the aim of the study was to investigate the effects of different interface materials and curing units on color changes in a resin cement material.materials and methods : three interface materials and different curing systems , quartz - tungsten - halogen and polywave and monowave light - emitting diode ( led ) light curing units , were studied at two - time intervals . 
 polystyrene strip was used as a control group . 
 all measurements were made on a white background for standard color measurement . according to the cie l*a*b * color space , the baseline color values of each specimen were measured . 
 differences between the measurements were calculated as e , l , a , and b . data were analyzed using analysis of variance ( anova ) and duncan 's tests (  = 0.05 ) with spss 20.0 software ( spss inc . , 
 chicago , il , usa ) . 
 anova revealed significance for interface materials and curing units and time for e ( p < 0.05).results : interaction between polymerizing units , material and time was not significant ( p > 0.05 ) . 
 monowave led exhibited significantly higher color changes than the other units ( [ p < 0.05 ] [ e 2.94  0.44 ] ) . 
 qth promoted composite specimens significantly less color change ( [ p < 0.05 ] [ e 0.87  0.41]).conclusion : this study concluded that color of resin cement used in the adhesion of indirect restorations was affected by curing device light and indirect restoration material type . 
INTRODUCTION
MATERIALS AND METHODS
Experimental design
Color measurements
Statistical analysis
RESULTS
DISCUSSION
CONCLUSION
Financial support and sponsorship
Conflicts of interest