polymerization shrinkage still harms the interfacial quality . when the shrinkage takes place under confinement , due to bonding to cavity walls , stresses on the bonding interface
the presence of gaps on the tooth / resin interface is considered as the first sign of restoration failure .
although polymerization shrinkage is considered the main cause of the adaptation problems of composite restorations , it is an inherent characteristic of the composite and directly related to its composition . moreover , the photoinitiator concentration can influence the shrinkage stress and consequently , the gap formation .
however , the effect of photoinitiator concentration in gap formation of composite restorations has not been evaluated .
the most frequently employed photoinitiator on dental composites is camphorquinone ( cq ) , which is a type ii photosensitizer ( it needs a reducing agent to produce free radicals ) with a light absorption peak near 468 nm . when cq is photoactivated , in the excited form , a pair of free radicals is produced by proton abstraction .
such a process can be made highly efficient by the formation of a complex between the photoexcited sensitizer and a reducing agent , such as a tertiary amine .
a higher concentration of cq can induce a quick and high generation of free radicals , yielding the production of a faster polymerization kinetics reaction and higher degree of conversion . as a result , mechanical properties of the material may also be enhanced .
however , studies have shown that there is an ideal level for the increase of cq concentration and above this level , the increase in photoinitiator concentration does not benefit the final degree of conversion .
conversely , if the concentration of photoinitiators were kept too low , resin composite could not be satisfactorily cured , which has been associated with poor biocompatibility , color stability , physical and mechanical properties , wear resistance and potentially early failure of the restoration .
in addition , a substantial problem is that cq is inherently yellow and has poor photobleaching , which means the yellow color remains after light irradiation .
it may cause problems in color matching to natural dental substrate and limits the quantity of cq that can be incorporated in dental composites .
a slow polymerization reaction allows a slow development of the composite stiffness and leads to better flow that can reduce the shrinkage stress .
studies have d emonstrated that the marginal adaptation of resin composites can be improved by light curing with low power density .
conversely , the application of an adequate energy dose for composite curing is necessary to achieve deep and complete polymerization of the material .
thus , modulated curing methods , such as soft - start ( ss ) and pulse - delay ( pd ) have been proposed .
the ss curing method enhances marginal sealing of cavo - surface margins of composite restoratives , especially due to initial lower viscosity of the composite , allowing a better material flow during the earlier stages of curing .
ss also leads to shrinkage , surface hardness and residual monomer concentration similar to conventional photoactivation , in situations with similar radiant exposure .
moreover , the pd photocuring method , which is a variation of the ss method , can also decrease the intensity of polymerization stress .
this curing method consists of an initial low irradiance exposure followed by a lag period in the absence of the curing light from 10 s to 5 min before a final high irradiance exposure .
the initial low energy density is enough to allow the start of the polymerization reaction of the composite .
the higher irradiance performed on the last stage of the curing method is responsible for ensuring similar physical and mechanical properties to the composite polymerized using the continuous light method .
some studies have associated this method with reduced gap formation , caused by the reduction of shrinkage stress .
however , it was also showed that pd method yields the formation of polymers with increased susceptibility to softening in ethanol , even though with similar monomer conversion in comparison with that produced using the continuous light curing technique .
the effects of modulated photoactivation method have been related to the composition of the resin material .
however , it is important to consider that most studies evaluating modulated photoactivation methods were conducted using commercial composites where fillers , resin matrix and photo initiation system differ considerably .
therefore , it seems to be more appropriate to use a resin composite model in which the variables could be controlled .
thus , the aim of this study was to determine the influence of photoinitiator concentration on superficial and internal adaptation of experimental composites photocured by different methods .
the first tested hypothesis was that a higher concentration of photoinitiator can increase gap formation .
monomer mixtures consisting of 65 wt% of bisgma ( bisphenol - a - glycidyl methacrylate ) and 35 wt% of triethylene glycol dimethacrylate ( tegdma ) were prepared .
the light - curing initiator system selected was cq as photoinitiator and dimethylaminoethyl methacrylate ( dmaema ) as the co - initiator ( proportion 1:1 by weight ) .
the light - curing initiator system ( cq / dmaema ) was thoroughly dissolved in the monomer matrix in the following concentrations : 0.5 wt% ( c1 - 0.25% cq/0.25% dmaema ) , 1.0 wt% ( c2 - 0.5% cq/0.5% dmaema ) and 1.5 wt% ( c3 - 0.75% cq/0.75% dmaema ) .
furthermore , the inhibitor butylated hydroxytoluene was added to the organic matrix in a concentration of 0.1 wt% to avoid spontaneous polymerization of the monomers .
the organic matrix was reinforced with silanized barium aluminum silicate glass fillers ( baalsi  average size : 0.5 m ) and silica ( sio2  average size : 0.04 m ) .
the fillers were gradually added in the resin matrix and mixed homogeneously to a 65 wt% loading . considering this filler content , 80 wt% were baalsi and 20 wt% were sio2 .
the formulation of the experimental composites used in this study was selected based on previous studies .
a total of 120 bovine incisors were selected , cleaned and stored in a 0.5% chloramine t solution at 4c for no more than a week .
roots were sectioned 1 mm below the cement - enamel junction using a double - face diamond saw ( k. g. sorensen , sao paulo , sp , brazil ) .
the buccal surface was ground on a water - cooled mechanical polisher ( metaserv 2000 , buehler , uk ltd , lake bluff , il , usa ) using 80 , 180 , 320 and 600 grit silicon carbide ( sic ) abrasive paper ( carbimet disc set , # 305178180 , buehler , uk ltd , lake bluff , il , usa ) in order to expose a flat enamel area of at least 36 mm .
these teeth were then observed on a stereomicroscope ( zeiss , manaus , am , brazil ) at 25 magnification , to investigate whether the enamel was completely removed . a cavity with 3 mm width , 3 mm length and 2 mm deep was prepared on the flattened surface using cylindrical diamond tips ( # 56 , kg .
sorensen , so paulo , sp , brazil ) mounted in a standard preparation device . in this condition ,
the cavity walls had a 90 angle with the dentin surface plane , while the internal cavity angles were rounded due to the design of the diamond tip used .
if any sign of pulp exposure was noticed during the cavity preparation , the specimen was discarded .
after cavity preparation , each specimen was restored using an etch - and - rinse adhesive system ( adper single bond 2 , batch # 3hr , 3m / espe , st .
paul , mn , usa ) , applied in accordance with manufacturer 's instructions : cavity was etched with 35% phosphoric acid gel ( scotchbond etchant , 3m / espe , st .
paul , mn , usa ) for 15 s , rinsed for 10 s and blot - dried .
the adhesive system was applied twice with a 5 s interval in between , dried carefully with air for 15 s in order to remove residual solvent ( observing a glossy surface ) and light cured for 20 s using a quartz tungsten halogen ( qth ) light ( xl 2500 , 3m / espe , st .
afterward , the teeth were randomly assigned into six groups ( n = 10 ) , according to the type of composite ( c1 , c2 and c3 ) and curing method ( high - intensity ; low - intensity ; ss ; pd ) .
paul , mn , usa ) was used for all curing methods using radiant exposure around 28 j / cm .
after the light curing procedures , the specimens were stored in distilled water at 37c for 24 h. after that , the specimens were finished under running water using 600- and 1200- grit sic abrasive paper ( carbimet disc set , # 305178180 , buehler , lake bluff , il , usa ) to expose and polished with 1 m and 0.5 m diamond pastes in a polish cloth under water .
specimens were ultrasonically cleaned for 5 min between finishing and polishing steps . in order to evaluate the marginal adaptation of the restorations
, impressions were taken with a low viscosity polyvinyl siloxane material ( aquasil , dentsply detrey , konstanz , germany ) and poured using an epoxy resin ( buehler , lake buff , il , eua ) to obtain replicas .
afterward , the replicas were gold - sputter coated ( balzers - scd 050 sputter coater , liechtenstein ) and observed by sem ( jeol , jsm-5600 lv , sem , japan ) for evaluation , measurement and classification of the cavity margins .
the classification of the margins was made at 200 magnification directly on the microscope monitor and the measurements were made by using a multi - point measuring device that allowed the observation of the entire perimeter of the restoration at 25 .
the perimeter of the restorations was measured and margins were recorded and classified according to the morphologically defined parameters previously described by kemp - scholte and davidson .
( 1 ) perfect margin : defined as a continuous , gap - free transition between filling and tooth substrate and ( 2 ) marginal gap : observed as gap formation with the loss of interfacial adhesion .
marginal gap formation was calculated and expressed as a percentage of the cavity perimeter of each specimen .
the length of the gap formed was calculated as a percentage of the entire margin length .
after the evaluation of superficial adaptation , the restorations were cut in slices ( 1 mm thick ) in the buccal - lingual direction , using a cutting machine ( isomet 1000 , buehler , lake bluff , il , usa ) to obtain three slices of each restoration for internal adaptation measurements .
thus , the sectioned slices were replicated and the internal margins of the restorations evaluated according to the procedures described for the superficial adaptation protocol .
for this reason , data were transformed   after transformation , data showed normal distribution according to kolmogorov - smirnov test .
transformed data ( gap % ) were subjected to two - way analysis of variance ( anova ) , considering the factors  composite  and  photoactivation method  and tukey 's test at a significance level of 5% .
all statistical analysis was executed in assistat beta 7.5 software ( assistat  statistic assistance by prof .
dr . francisco de assis santos e silva , deag - ctrn  university of campina grande  pb 
monomer mixtures consisting of 65 wt% of bisgma ( bisphenol - a - glycidyl methacrylate ) and 35 wt% of triethylene glycol dimethacrylate ( tegdma ) were prepared .
the light - curing initiator system selected was cq as photoinitiator and dimethylaminoethyl methacrylate ( dmaema ) as the co - initiator ( proportion 1:1 by weight ) .
the light - curing initiator system ( cq / dmaema ) was thoroughly dissolved in the monomer matrix in the following concentrations : 0.5 wt% ( c1 - 0.25% cq/0.25% dmaema ) , 1.0 wt% ( c2 - 0.5% cq/0.5% dmaema ) and 1.5 wt% ( c3 - 0.75% cq/0.75% dmaema ) .
furthermore , the inhibitor butylated hydroxytoluene was added to the organic matrix in a concentration of 0.1 wt% to avoid spontaneous polymerization of the monomers .
the organic matrix was reinforced with silanized barium aluminum silicate glass fillers ( baalsi  average size : 0.5 m ) and silica ( sio2  average size : 0.04 m ) .
the fillers were gradually added in the resin matrix and mixed homogeneously to a 65 wt% loading . considering this filler content , 80 wt% were baalsi and 20 wt% were sio2 .
the formulation of the experimental composites used in this study was selected based on previous studies .
a total of 120 bovine incisors were selected , cleaned and stored in a 0.5% chloramine t solution at 4c for no more than a week .
roots were sectioned 1 mm below the cement - enamel junction using a double - face diamond saw ( k. g. sorensen , sao paulo , sp , brazil ) .
the buccal surface was ground on a water - cooled mechanical polisher ( metaserv 2000 , buehler , uk ltd , lake bluff , il , usa ) using 80 , 180 , 320 and 600 grit silicon carbide ( sic ) abrasive paper ( carbimet disc set , # 305178180 , buehler , uk ltd , lake bluff , il , usa ) in order to expose a flat enamel area of at least 36 mm .
these teeth were then observed on a stereomicroscope ( zeiss , manaus , am , brazil ) at 25 magnification , to investigate whether the enamel was completely removed . a cavity with 3 mm width , 3 mm length and 2 mm deep was prepared on the flattened surface using cylindrical diamond tips ( # 56 , kg .
sorensen , so paulo , sp , brazil ) mounted in a standard preparation device . in this condition ,
the cavity walls had a 90 angle with the dentin surface plane , while the internal cavity angles were rounded due to the design of the diamond tip used .
if any sign of pulp exposure was noticed during the cavity preparation , the specimen was discarded .
after cavity preparation , each specimen was restored using an etch - and - rinse adhesive system ( adper single bond 2 , batch # 3hr , 3m / espe , st .
paul , mn , usa ) , applied in accordance with manufacturer 's instructions : cavity was etched with 35% phosphoric acid gel ( scotchbond etchant , 3m / espe , st .
paul , mn , usa ) for 15 s , rinsed for 10 s and blot - dried .
the adhesive system was applied twice with a 5 s interval in between , dried carefully with air for 15 s in order to remove residual solvent ( observing a glossy surface ) and light cured for 20 s using a quartz tungsten halogen ( qth ) light ( xl 2500 , 3m / espe , st .
afterward , the teeth were randomly assigned into six groups ( n = 10 ) , according to the type of composite ( c1 , c2 and c3 ) and curing method ( high - intensity ; low - intensity ; ss ; pd ) . cavities were bulk filled with one of the tested experimental composites .
curing methods are described in table 1 . the same qth curing unit ( xl 2500 , 3m / espe , st .
paul , mn , usa ) was used for all curing methods using radiant exposure around 28 j / cm .
after the light curing procedures , the specimens were stored in distilled water at 37c for 24 h. after that , the specimens were finished under running water using 600- and 1200- grit sic abrasive paper ( carbimet disc set , # 305178180 , buehler , lake bluff , il , usa ) to expose and polished with 1 m and 0.5 m diamond pastes in a polish cloth under water .
specimens were ultrasonically cleaned for 5 min between finishing and polishing steps . in order to evaluate the marginal adaptation of the restorations
, impressions were taken with a low viscosity polyvinyl siloxane material ( aquasil , dentsply detrey , konstanz , germany ) and poured using an epoxy resin ( buehler , lake buff , il , eua ) to obtain replicas .
afterward , the replicas were gold - sputter coated ( balzers - scd 050 sputter coater , liechtenstein ) and observed by sem ( jeol , jsm-5600 lv , sem , japan ) for evaluation , measurement and classification of the cavity margins .
the classification of the margins was made at 200 magnification directly on the microscope monitor and the measurements were made by using a multi - point measuring device that allowed the observation of the entire perimeter of the restoration at 25 .
the perimeter of the restorations was measured and margins were recorded and classified according to the morphologically defined parameters previously described by kemp - scholte and davidson .
( 1 ) perfect margin : defined as a continuous , gap - free transition between filling and tooth substrate and ( 2 ) marginal gap : observed as gap formation with the loss of interfacial adhesion .
marginal gap formation was calculated and expressed as a percentage of the cavity perimeter of each specimen .
the length of the gap formed was calculated as a percentage of the entire margin length .
after the evaluation of superficial adaptation , the restorations were cut in slices ( 1 mm thick ) in the buccal - lingual direction , using a cutting machine ( isomet 1000 , buehler , lake bluff , il , usa ) to obtain three slices of each restoration for internal adaptation measurements .
thus , the sectioned slices were replicated and the internal margins of the restorations evaluated according to the procedures described for the superficial adaptation protocol .
in order to evaluate the marginal adaptation of the restorations , impressions were taken with a low viscosity polyvinyl siloxane material ( aquasil , dentsply detrey , konstanz , germany ) and poured using an epoxy resin ( buehler , lake buff , il , eua ) to obtain replicas .
afterward , the replicas were gold - sputter coated ( balzers - scd 050 sputter coater , liechtenstein ) and observed by sem ( jeol , jsm-5600 lv , sem , japan ) for evaluation , measurement and classification of the cavity margins .
the classification of the margins was made at 200 magnification directly on the microscope monitor and the measurements were made by using a multi - point measuring device that allowed the observation of the entire perimeter of the restoration at 25 .
the perimeter of the restorations was measured and margins were recorded and classified according to the morphologically defined parameters previously described by kemp - scholte and davidson .
( 1 ) perfect margin : defined as a continuous , gap - free transition between filling and tooth substrate and ( 2 ) marginal gap : observed as gap formation with the loss of interfacial adhesion .
marginal gap formation was calculated and expressed as a percentage of the cavity perimeter of each specimen .
the length of the gap formed was calculated as a percentage of the entire margin length .
after the evaluation of superficial adaptation , the restorations were cut in slices ( 1 mm thick ) in the buccal - lingual direction , using a cutting machine ( isomet 1000 , buehler , lake bluff , il , usa ) to obtain three slices of each restoration for internal adaptation measurements .
thus , the sectioned slices were replicated and the internal margins of the restorations evaluated according to the procedures described for the superficial adaptation protocol .
, data were transformed   after transformation , data showed normal distribution according to kolmogorov - smirnov test .
transformed data ( gap % ) were subjected to two - way analysis of variance ( anova ) , considering the factors  composite  and  photoactivation method  and tukey 's test at a significance level of 5% .
all statistical analysis was executed in assistat beta 7.5 software ( assistat  statistic assistance by prof .
dr . francisco de assis santos e silva , deag - ctrn  university of campina grande  pb 
superficial gap formation ( % ) means and standard deviations for all photoactivation methods are listed in table 2 . according to two - way anova , for superficial marginal adaptation , only the factor 
the factor  photoactivation method  as well as the interaction between the two factors was not statistically significant .
in addition , the composite c1 presented higher gap formation when compared to c2 or c3 .
superficial gap formation ( % ) means and standard deviations for all photoactivation methods internal gap formation ( % ) means and standard deviations for all photoactivation methods are listed in table 3 . as observed for superficial marginal adaptation , only the factor composite showed a significant difference , regardless of the photoactivation method . in the accompanying results of marginal gap formation , there was no difference between the curing methods , regarding the composite used .
in addition , the composite c1 presented higher gap formation when compared to c2 or c3 . internal gap formation ( % ) means and standard deviations for all photoactivation methods
photoinitiator concentration is a fundamental parameter that determines the polymerization characteristics of a resin composite .
this concentration varies among the commercial composites and its effects on marginal and internal adaptation are not still completely elucidated .
taira et al . found cq concentrations that range from 0.17 to 1.03 wt% of the resin and shintani et al . reported a range of 0.03 - 0.09 wt% of the dental composite . in this study ,
the concentration of cq ranged from 0.25 to 0.75 wt% of the resin matrix ( which corresponds to 0.087 - 0.26 wt% of the composite ) and it has affected superficial and internal gap formation . however , this effect was opposite that expected : the composite with lower concentration of initiators ( c1 ) presented a higher gap percentage then composites with a higher concentration ( c2 and c3 ) , rejecting the first hypothesis .
alonso et al . , using the same composite formulation as the present work , found a significant reduction of monomer conversion when c1 ( 0.5% of cq ) was compared to c3 ( 1.5% of initiator ) , showing that the higher the initiator concentration , the higher the conversion degree . in this sense , it was expected that c3 would present higher gap formation , since a higher degree of conversion means higher shrinkage , higher elastic modulus and higher stress at the bonding interface .
the poorly polymerized composite would not adequately bond to the adhesive resin , allowing gap formation , as observed in figure 1 .
the failure occurs between the restorative composite and the adhesive layer , what could be related to the poor polymerization of the composite .
rc : restorative composite , ad : adhesive layer photoinitiator concentration of a resin composite must be sufficient to allow an adequate polymerization .
an insufficient polymerization would affect biocompatibility of the composite , due to residual monomer release .
showed that cq and tegdma are released to the aqueous substrate by unsatisfactory polymerized resin materials and can damage oral tissue when presented in high concentration .
in addition , bonding problems , like the ones demonstrated in the present study [ figure 1 ] , can also occur in composite restorations with inadequate polymerization . however , it is important to consider that cq / amine concentration should be as low as possible due to its yellowing effect , which can cause difficulty in matching dental color and also the tendency to internal discoloration yielded by amine .
so , considering the composites evaluated in this study , 1% of cq / amine ( c2 ) could be considered to be adequate since it is sufficient to guarantee a satisfactory polymerization and marginal seal , since c2 and c3 present similar gap formation [ tables 2 and 3 ] and a similar degree of conversion , according to alonso et al .
esthetics and bond durability of these restorations are directly influenced by the marginal seal . in general ,
the origin of gaps has been related to three main factors : contraction stress of composite restoratives , bonding failures , and external mechanical or thermal stress . in the present study , specimens were not submitted to any mechanical or thermal stress and bonding procedures were standardized for all groups .
this condition enabled the evaluation of gap formation as a result of contraction stress of the experimental composites with different photoinitiator concentrations and different curing methods .
therefore , it is important to state that besides the care taken during specimen preparation , gap formation studies usually show high variability of the data , as inside each group some specimens present a perfect seal while others present a high percentage of gaps .
data variability in gap formation studies can be attributed to the great variability of the dental substrate , especially considering bonding to dentin .
for this reason , it is important to use a minimum of 10 specimens per group .
the majority of the specimens showed a perfect seal of the superficial enamel margins in the present study , as observed in figure 2 .
enamel presents high mineral content , allowing good and stable bond strength when the etch - and - rinse technique is used .
these results are in accordance with the studies of correr et al . and alonso et al .
regarding internal adaptation , gap formation was more frequently observed at the axiopulpal angle and pulpal wall .
misfit at the angle may be attributed to the more complex accommodation of the composite in this region , as observed in figure 3 .
in addition , stress concentration on the cavity angle and on defects at the adhesive layer contribute to misfit . on the pulpal wall
, gaps can be attributed to the greater complexity of adhesion due to the higher density and diameter of dentin tubules , decreasing the intertubular area that is essential for bonding .
d : dentin , rc : restorative composite another studied factor was the curing protocol .
it was hypothesized that modulated photoactivation methods could reduce gap formation ( second hypothesis ) .
modulated curing methods did not improve marginal or internal adaptation of composite restorations , corroborating lopes et al . and amaral et al .
irradiance can directly influence the polymerization rate , which is related to shrinkage stress development .
the higher polymerization rate decreases composite flow , leading to more frequent gap formation on the tooth / restoration interface due to the fast development of rigidity , reducing the viscoelastic period .
thus , the increase of elastic modulus added to polymerization shrinkage yields stress development at the bonding interface , which has been related to marginal adaptation failure of composite restorations . in this sense , the low irradiance method or the modulated ones can be convenient by reducing the polymerization rate , which favors a slower contraction stress development . on the other hand , this effect was not able to reduce gap formation in the present study .
this suggests that the effects of reduced curing rates on contraction stress are limited and significant reductions in stress can be verified only after the curing rate drops below a certain threshold , as stated by braga et al . in this sense , it could be speculated that the determinant factor for gap formation would be the energy dose applied , so that composites photocured with similar energy density presents similar gap formation .
this speculation is supported by others studies considering that the total volumetric contraction of a composite and maximum stress of this system depend on monomer conversion , which is energy - dose - dependent .
alonso et al . showed that the degree of conversion of the composites tested here was similar for all photoactivation methods as the same energy dose was applied .
some studies have shown that the efficacy of modulated methods vary according to composite formulation . in this way
another disadvantage of these methods is the decrease of cross - link density on polymer network , caused by the lower irradiance application , yielding the composite matrix to be more susceptible to degradation . in accordance with the results of the present study ,
resin composites with different photoinitiator concentration show different marginal and internal adaptation features , perhaps due to the polymerization behavior .
however , it is not affected by modulation of light intensity ; composite restoration photocured by modulated methods presents similar marginal quality as the energy dose was kept constant .
thus , only the understanding of the problem statement , allied to the development of techniques to reduce their consequences , would help clinicians to obtain the maximum benefits of resin composite restorations in clinical practice .
therefore , higher photoinitiator concentrations in composite allow better marginal seal for the tested conditions .
 objective : the aim of this study was to evaluate the influence of photoinitiator concentration on marginal and internal adaptation of composites photocured by modulated methods.materials and methods : composites based on bisgma / triethylene glycol dimethacrylate and 65 wt% of filler were prepared with different concentrations of camphorquinone / amine ( c1 - 0.5% , c2 - 1% , c3 - 1.5% ) . cavities were prepared ( 3 mm  3 mm  2 mm ) on the buccal surface of 120 bovine incisors and the adhesive system adper single bond 2 was applied following manufactures instruction . 
 specimens were then distributed according to type of composite ( c1 , c2 , c3 ) and photoactivation method ( high - intensity  750 mw / cm2 for 40 s ; low intensity  150 mw / cm2 for 200 s ; soft - start  150 mw / cm2 for 10 s + 750 mw / cm2 for 38 s ; pulse - delay  150 mw / cm2 for 10 s + 3 min dark + 750 mw / cm2 for 38 s ) . 
 superficial and internal margins were analyzed by scanning electron microscopy , using the epoxy replica technique . 
 the length of gaps was expressed as a percentage of the total length of the margins . 
 data were submitted to two - way analysis of variance and tukey 's test (  = 0.05).results : modulated curing methods did not influence gap formation regarding both superficial and internal adaptation . 
 the composite with the lower initiator concentration ( c1 ) presented higher gap formation when compared with those with higher concentrations ( c2 and c3).conclusion : modulated photoactivation methods did not reduce gap formation for the experimental composite restorations evaluated . 
 however , higher photoinitiator concentrations promote better marginal seal . 
INTRODUCTION
MATERIALS AND METHODS
Formulation of experimental composites
Specimen preparation
Evaluation of marginal and internal adaptation by scanning electron microscopy
Statistical analysis
RESULTS
DISCUSSION
CONCLUSION