resin - bonded fixed partial dentures ( fpds ) are inexpensive and there are more conservative treatment options for replacing missing teeth [ 1 , 2 ] .
metal resin - bonded fpds continue to decrease demand because of the existing several disadvantages .
while metal substrate is durable and stiff , it has considerable esthetic problems such as discoloration of the gingiva and abutment tooth  graying. the other drawbacks may occur with these prostheses ; namely , the potential for alloy hypersensitivity , retainer fracture , and loss of attachment from bridge abutments , corrosion and health risks to laboratory staff [ 35 ] . given thes problems ,
composite materials have been a subject to attention for metal - free fpds after the advent of using fibers as frameworks to reinforce them .
in addition , these economically feasible restorations have proper esthetics , low weight and favorable elastic modulus [ 2 , 4 , 6 ] .
conversely , color stability and wear resistance are remarkable potentials of ceramics , but these materials have some liabilities such as brittleness , disability of adhesion to tooth structure and having the potential to damage the unrestored opposing teeth [ 3 , 7 ] . based on these backgrounds , currently frc
are not only used for crowns and inlays , but also for a variety of esthetic restorations in clinics , for example the suprastructure of implants and fpds .
different kinds of fiber materials exist in the market , but at present polyethylene and glass fibers are the most popular . to date
, several studies have evaluated the mechanical properties ( flexural strength and fracture strength ) of frc [ 4 , 6 , 8 , 10 ] , but a few studies have been conducted to find out and understand the color properties of frc [ 1114 ] .
.   stated significant color change between non - frc and frc restorations with a decrease in lightness .
tuncdemir and aykent   measured the effect of fiber reinforcement on the color stability of composite resin .
they concluded that an everstick net fiber - reinforced anterior composite combination exhibits trace color change , but the other frc groups reveal slight color change without accelerated aging .
after accelerated aging , they found no significant color difference between frc and non - frc restorations .
these studies analyze different types of composites with varying types of fibers and the results are different [ 1114 ] .
so , this study investigated the influence of glass and polyethylene fibers and veneering composites ( direct and indirect ) in the frc system on color differences . the null hypothesis assumed in this study was that the fibers and veneering composites did not influence the color differences of fiber - reinforced composites and non - fiber - reinforced composites .
two types of fiber - reinforced system and veneering composite were investigated in the present study ( table 1 ) .
specimen preparation : using two stainless steel molds , 13 mm in diameter and 1 mm or 3 mm height , 40 cylindrical specimens were prepared ( fig 1 ) .
composite resin veneers were packed into a mold on a glass plate . after packing , a second glass plate was placed over the mold , followed by compression to run off the excess composite resin into the escape area .
the z250 cylinders were light - cured using the light curing unit ( bluephase ivoclar vivadent , austria ) for 20s with an output of 1,000 mw / cm2 light intensity .
the output of the curing light was checked with a radiometer ( 1,000 mw / cm2 ) .
but the initial curing of gradia specimens was done according to the recommendations by gc steplight sl - i for 10s .
after light - curing , the top glass plate was removed and the specimens were then removed from the mold .
twenty ( 1 mm height ) cylinders of each base composite were prepared according to the manufacturer`s recommendations ( 1618 ) .
a 20-second duration cure and handling of glass slab was similar to the veneering composite .
these cylinders were entered in the thicker mold . in the cylinders of nulite f base composites ,
polyethylene fibers were cut in 10 mm size , impregnated in resist resin and embedded on thin layers of nulite f composites and post - cured 20 seconds after , then a small layer of nulite f composite covered the fiber .
subsequently , the cylinders of each veneering composite material were carried to a thicker mold and pressed with a glass slab to remove the excess composite . in
z250 veneering composite , the specimens were light cured using the blue phase ( ivoclar vivadent , austria ) visible - light - curing unit for 20s with 1000mw / cm2 light intensity ( g1 ) and gradia indirect completion of curing was done in gc labolight lv- ,  cure unit ( g3 ) . in adhesive c cylinders ,
the proper pieces ( 10 mm ) of glass fibers were cut , placed on a thin layer of adhesive c composite and cured for 20 seconds .
veneering composites in ( g5 ) and ( g7 ) were used in a similar manner as explained above for ( g1 ) and ( g3 ) , respectively . in g2 and g6 ,
z250 cylinders were placed in thicker molds and the remaining spaces in the molds were packed with nulite f and adhesive c , respectively . for g4 and g8 , gradia indirect cylinders were placed in thicker molds and the remaining spaces in the molds were packed with nulite f and adhesive c , respectively .
the color parameters of the disks were measured over a white background using a reflection spectrophotometer ( color - eye 7000a , gretagmacbeth , new windsor , ny , usa ) , according to cie l * a * b * color system relative to the standard illumination tungsten d65 lamp . prior to each of the color measurement series , a calibration was done based on the manufacturers instruction .
here , a * , b * represented the colors on the green - red and blue - yellow axes , respectively and l * represented lightness scale .
the color differences ( e ) between the groups were calculated by equation : e * = [ ( l*)2 + ( a*)2 + ( b*)2 ] 1/2 .
the extent of significant color difference was evaluated by two - way analysis of variance ( anova ) combined with tukey s hsd at the 0.05 level of significance .
the color parameters of the disks were measured over a white background using a reflection spectrophotometer ( color - eye 7000a , gretagmacbeth , new windsor , ny , usa ) , according to cie l * a * b * color system relative to the standard illumination tungsten d65 lamp . prior to each of the color measurement series ,
here , a * , b * represented the colors on the green - red and blue - yellow axes , respectively and l * represented lightness scale .
the color differences ( e ) between the groups were calculated by equation : e * = [ ( l*)2 + ( a*)2 + ( b*)2 ] 1/2 .
the extent of significant color difference was evaluated by two - way analysis of variance ( anova ) combined with tukey s hsd at the 0.05 level of significance .
the values of l * , a * , b * , and the differences in color ( e ) are indicated in table 3 .
application of two - way anova revealed that l * , a * and b * were significantly affected by the veneering composites ( p<0.05 ) , and by the fibers ( p<0.05 ) .
the negative sign of l * shows that incorporation of fibers into composite in experimental groups made them darker than the controls , except in g7 .
g1 exhibited a red - blue shift ( a*= 0.98 , b*= 0.93 and the other groups ( g3 : b*= 1.54 , a*= 0.37 , g5 : b*= 1.82 , a*= 0.42 , g7 : b*= 0.77 , a*= 0.61 ) a green - blue shift .
pairwise comparison by tukey s hsd test confirmed the significant statistical differences in a * and l * between g1 and g3 , between g1 and g5 and between g3 and g7 ( p<0.05 ) .
furthermore , pairwise comparison by tukey s hsd test indicated no significant statistical differences in b * between g1 and g3 and between g3 and g7 ( p<0.05 .
tukey s hsd indicated statistical differences in b * between g1 and g5 ( p=0.004 ) .
the null hypothesis was verified because no significant differences between the groups studied in e were observed .
e values of all groups vary from 1.91 to 2.32 , which were lower than the threshold of clinical acceptable color difference . clinical color - matching tolerance between the tooth and restorations may be categorized according to e , based on clinical studies below : ( e:0=perfect , e:0.51=excellent , e:12= good , e:23.5=clinically acceptable , e :> 3.5=mismatch ) . according to these classifications , clinical color match in g3 and
these results were in disagreement with the findings of a previous study , whereby incorporation of fibers in the composite caused color mismatches that were not clinically acceptable ( 5.29 < e
< 8.19 ) . they only investigated the incorporation of glass - fiber in single - layer and double - layer in direct dpi curex microhybrid composite ( shade a1 ) and the overall thickness of all specimens were 2 mm .
one possible explanation about the different results is that the greater amount of veneering composite for covering specimens may reduce the effect of fibers .
since the type of fibers and veneering composites were clearly different from the current study , direct comparison was impossible .
tuncdemir and aykent   demonstrated that the types of composite and fiber materials used explained the color difference between frc and non - frc restorations without aging .
they have reported less color differences ( 0.32<e<1.03 ) between groups than those observed ( 1.91<e<2.32 ) in the present study .
differences in the range of e in our study and the previous study   may be due to the use of these base composites .
.   showed that reproduction of the similar color to the veneering composite was possible in all experimental groups ( except in the case of fibrekor ) .
as indicated in earlier studies , the key factors affecting the optical properties ( color change ) in composites are as follows : ( i ) size , shape and filler content , ( ii ) type of polymerization , ( iii ) organic matrix , and ( iv ) thickness of composite .
z250 is a microhybrid composite that contains bis - gma , udma and bisema monomers and zirconium / silicon fillers ( 60% volume , 0.01 to 3.5 micrometers ) .
gradia is a micro - filled udma - based composite that is mixed with a fine particle glass filler , silica nanofiller and prepolymerized filler ( 75 wt % ) .
although compositions and the method of polymerization of the studied composites varied , these differences had no significant effect on color changes .
these results could be linked to the interaction between fibers and composites . on the other hand , this in vitro finding may be different from the clinical circumstances .
meanwhile , all specimens employed as the model of color assessment in this study were flat and this led to the omission of surface morphology - associated color characteristics . in this study ,
the comparison between the observations of the human eyes or conventional methods showed that spectrophotometer offered a 33% enhancement in precision . in the present study ,
the visual inspection of skillful people and other viewers did not verify the results of the spectrophotometer .
conversely , one review highlighted that visual and instrumental methods completed each other and could serve as a guide towards predictable cosmetic results .
thus , evaluation of other color coordinates is essential . with respect to the values of l , the incorporation of fibers caused a decrease of the l * values ( samples became darker ) for all the experimental groups except in g7 .
more negative l values between the polyethylene experimental groups and the glass experimental groups possibly originated from the greater thickness of the polyethylene fibers , and also from the difference in the nature of fibers between these two types of fibers , which could result in a slighter amount of beam reflected back ; back - scattered out of the specimen or absorbed . in the present state of our knowledge , we did not find any justification for the result in g7 .
there is a color shift of experimental groups towards the shorter wavelength regions , which is in the direction of green and blue , except in g1 .
these findings are inconsistent with the previous study because they saw a shift toward the longer wavelength ( red - yellow ) .
perhaps the interactions between fibers and veneering composite in all experimental groups ( except in g1 ) were followed by shifts toward the lower wavelength area . on the basis of the limited information on the interaction modality of the optical properties of the fiber and the veneering composite in literature
, we did not suggest any explanation responsible for the shift to the red axis in g1 .
it should be kept in mind that the result of the present article is based on an in vitro condition .
meanwhile , this study only investigated the combination application of the two types of fibers and veneering composites .
therefore , it is intricate to generalize the outcome of this study to all frc systems and clinical conditions .
the clinical conditions may worsen the status of the continuing test and may affect the results .
several variations exist between clinic and test conditions that may influence the color change of frcs including : ( i ) the investigated specimens had flat surfaces ; whereas , in clinical status the anatomical surfaces were reproduced on prosthesis , ( ii ) the preparation of the abutment teeth , ( iii ) oral conditions ( loading , temperature changes , saliva , diet and oral hygiene ) .
consequently , a more complete strategy should be developed to assess the oral environment effects on color changes of frcs .
with the limitations of the current study , veneering composites and the fibers showed no statistical effect on the color change ( e ) of frcs .
all experimental groups were darker compared with the control groups except for the glass fibers - gradia indirect group .
 objective : color match between fiber - reinforced composite ( frc ) restorations and teeth is an imperative factor in esthetic dentistry . 
 the purpose of this study is to evaluate the influence of veneering composites and fibers on the color change of frc restorations.materials and methods : glass and polyethylene fibers were used to reinforce a direct microhybrid composite ( z250 , 3 m espe ) and a microfilled composite ( gradia indirect , gc ) . there were eight experimental groups ( n=5 disks per group ) . 
 four groups were used as the controls ( non - frc control ) and the others were used as experimental groups . 
 cielab parameters ( l * , a * and b * ) of specimens were evaluated against a white background using a spectrophotometer to assess the color change . the color difference ( e * ) and 
 color coordinates were ( l * , a * and b * ) analyzed by two - way anova and tukey s test.results:both types of composite and fiber influenced the color parameters ( l * , a * ) . 
 the incorporation of fibers into the composite in the experimental groups made them darker than the control groups , except in the gradia indirect+ glass fibers group . 
 b * is affected by types of fibers only in direct fiber reinforced composite . 
 no statistically significant differences were recognized in e * among the groups ( p>0.05).conclusion : the findings of the present study suggest that the tested frc restorations exhibited no difference in color in comparison with non - frc restoration . 
 hence , the types of veneering composites and fibers did not influence the color change ( e * ) of frc restorations . 
INTRODUCTION
MATERIALS AND METHODS
Color measurements:
Statistical analysis:
RESULTS
DISCUSSION
CONCLUSION