endodontically treated teeth presenting partial or total destruction of crowns require 
 reconstruction to create a core to provide mechanical conditions for the indirect 
 restoration to be fixed and remain in function for a long period .
the materials most 
 employed for that purpose are composite resins and cast metallic posts .
resin - based cements have been widely employed for cementation of metaloceramic or 
 ceramic crowns due to their adhesive capacity to both tooth structure and restoration , 
 combined with esthetic and mechanical properties .
since the success of cementation depends on the 
 achievement of a strong and long - lasting bond among cement , restoration and tooth 
 structure , the strength of 
 such adhesion procedure is directly proportional to the adequate curing of the cement 
 and is crucial to achieve optimal physical and mechanical properties and satisfactory 
 clinical performance . several factors may influence the curing of the cement used for luting ceramic crowns . 

these include : the composition , thickness , opacity and shade of the ceramic , which may 
 reduce the light transmission and consequently affect the light - curing of the 
 cement ; characteristics of substrates and luting 
 agent ; incompatibility of simplified adhesive systems with self- or 
 dual - cured resin cements ; and permeability of simplified adhesives , which ultimately 
 compromises the bonding between the cement and the adhesive . considering the lack of information in the literature on the possible influence of the 
 type of material used as coronal reconstruction on the physical properties of the cement 
 used for luting ceramic crowns
, this study evaluated the microhardness of the resin 
 cement variolink ii along the cement line in the cervical , medium and 
 occlusal thirds on the buccal and lingual aspects , and on the occlusal surface , when 
 used for luting ceramic crowns on different substrates ( dentin , metal , and composite 
 resin ) , after 7 days and 3 months of water storage .
the null hypotheses tested were that 
 microhardness would not be influenced by the region of prepared surfaces , coronal 
 substrate , and water storage .
thirty human third molars were embedded with plaster in plastic cylinders with the 
 cementoenamel junction approximately 3 mm above the top of the cylinder .
the teeth were 
 prepared with diamond burs for full - ceramic crowns with a shoulder of 1.2 mm with 
 internal rounded angles , and axial reduction of 1.5 mm with 6 to 10 convergence angle 
 was performed .
occlusal reduction was performed resulting in an axial height of 4.0 mm 
 ( figure 1a ) .
they were randomly divided into 3 
 groups ( n=10 ) as follows : group d- the prepared crown surface was kept in dentin ; group 
 m- the crowns were sectioned at the level of the cementoenamel junction and the core was 
 modeled in acrylic resin duralay ( reliance dental mfg .
co. worth , illinois , usa ) , cast 
 in aluminum - copper alloy , and luted with zinc phosphate ( s. s. white artigos dentrios 
 ltda . , rio de janeiro , rj , brasil ) ; group r- the crowns were sectioned as in group m , 
 and filling of the cores was performed with composite resin filtek z250 ( 3 m 
 espe , st paul , mn , usa ) by the incremental technique .
light irradiation was obtained 
 from a quartz tungsten halogen ( qth ) device v.i.p .
junior ( bisco , schaumburg , il , usa , 
 500 mw / cm ) for 20 s for each increment , and 40 s for the last one .
the reduction , 
 convergence and height of axial walls followed the same principles described above for 
 group d. scheme of experimental stages : a ) prepared tooth ; b ) wax relief ; c ) cemented 
 crown ; d ) sectioned crown exhibiting microhardness measurements along the cement 
 line and ceramic thicknesses silicon molds were made of all sound tooth crowns before preparation .
these were then 
 used to guide the construction of the ceramic crowns to a thickness of 1.5 mm on the 
 axial walls and 2.0 mm on the occlusal surfaces . on the center of buccal , occlusal and lingual aspects of the prepared surfaces ,
a relief 
 was made in wax with approximate thickness of 0.25 mm and width of 2.0 mm to purposely 
 allow for a thicker cement line and permit the microhardness tests ( figure 1b ) .
impressions of the preparations were taken with 
 polyvinyl siloxane express ( 3 m espe , st paul , mn , usa ) and cast with type iv 
 plaster .
the crowns were fabricated from the type iv models with monolithic ceramic ips 
 e.max press lt ( ivoclar vivadent , schaan , liechtenstein ) , shade a2 , 
 following the manufacturer 's instructions . after fitting adjustments
, all crowns were luted with variolink ii cement ( figure 2 ) , following the procedures described in 
 figure 3 , and submitted to a static load of 5 
 kg during light - curing process .
after removal of the excess of cement , light curing was 
 performed on the buccal , lingual and occlusal surfaces , for 40 s on each surface ( figure 1c ) .
the specimens were then stored in 
 lightproof flasks , immersed in deionized water and kept at 37c .
the specimens were 
 randomly divided in subgroups of 5 for each substrate and sectioned either after 7 days 
 or 3 months of water storage .
chemical composition of the resin cement and adhesive system the teeth were removed from the embedding cups and transversely sectioned below the 
 crown margins using a diamond disc ( buehler , lake bluff , il , usa ) under constant 
 irrigation .
the crowns were then sectioned in a buccolingual direction , at the center 
 of the relief area , to expose the cement line . to facilitate positioning of the 
 specimen in the microhardness tester , a second parallel section limited to ceramic 
 was made to keep the surface to be analyzed perpendicular to the indenter .
the cut surface was sequentially polished with 600- and 1200-grit sic paper , followed 
 by 1-m diamond paste on a cloth , under constant irrigation . between each polishing 
 step ,
the specimens were rinsed with water for 30 s and ultrasonicated in deionized 
 water for 2 min .
the polished crown sections were kept in moist gauze in lightproof 
 flasks until tested .
measurements were performed on a shimadzu microhardness tester hmv-2,000 ( shimadzu 
 corporation - kyoto , japan ) with knoop indenter under a static load of 50 g for 10 s. 
 indentations were made from cervical to occlusal surface in 0.5 mm intervals along 
 the cement line ( figure 1d ) .
the hardness was 
 expressed as a knoop hardness number ( khn ) , and at the end of measurements the 
 average microhardness values were obtained for the cervical , medium and occlusal 
 thirds and occlusal surface .
data were analyzed by three - way anova ( substrates , thirds / occlusal surface , and 
 storage ) , and two - way anova was applied ( substrate / storage ) .
the teeth were removed from the embedding cups and transversely sectioned below the 
 crown margins using a diamond disc ( buehler , lake bluff , il , usa ) under constant 
 irrigation .
the crowns were then sectioned in a buccolingual direction , at the center 
 of the relief area , to expose the cement line . to facilitate positioning of the 
 specimen in the microhardness tester , a second parallel section limited to ceramic 
 was made to keep the surface to be analyzed perpendicular to the indenter .
the cut surface was sequentially polished with 600- and 1200-grit sic paper , followed 
 by 1-m diamond paste on a cloth , under constant irrigation . between each polishing 
 step ,
the specimens were rinsed with water for 30 s and ultrasonicated in deionized 
 water for 2 min .
the polished crown sections were kept in moist gauze in lightproof 
 flasks until tested .
measurements were performed on a shimadzu microhardness tester hmv-2,000 ( shimadzu 
 corporation - kyoto , japan ) with knoop indenter under a static load of 50 g for 10 s. 
 indentations were made from cervical to occlusal surface in 0.5 mm intervals along 
 the cement line ( figure 1d ) .
the hardness was 
 expressed as a knoop hardness number ( khn ) , and at the end of measurements the 
 average microhardness values were obtained for the cervical , medium and occlusal 
 thirds and occlusal surface .
data were analyzed by three - way anova ( substrates , thirds / occlusal surface , and 
 storage ) , and two - way anova was applied ( substrate / storage ) .
data were analyzed by three - way anova ( substrates , thirds , and storage ) .
considering 
 each factor independently , the substrates showed significant differences ( p=0.000 ) , 
 without differences for factors storage ( p=0.573 ) or thirds ( p=0.231 ) ( table 1 )
since the analysis did not reveal 
 significant differences between thirds , the values of substrate and storage were 
 submitted to two - way anova , which showed significant effects of core materials 
 ( p<0.001 ) .
hardness values were significantly lower when crowns were cemented on 
 resin cores and measured after 7 days of storage ( p=0.007 ) ( table 2 , figure 4 ) .
knoop hardness number ( standard deviation ) at cervical , middle , occlusal thirds 
 and occlusal surface according to the conditions substrate / time storage n=5 specimens in each group .
equal capital letters indicate lack of 
 statistically significant difference ( analysis of storage ) ; equal lowercase 
 letters indicate lack of statistically significant difference ( analysis of 
 substrate ) ; equal symbols indicate lack of statistically significant difference 
 ( analysis of thirds ) .
p<0.05 knoop hardness number ( standard deviation ) of samples according to the conditions 
 substrate / time storage n=5 specimens in each group .
equal capital letters indicate lack of 
 statistically significant difference ( analysis by row ) ; equal lowercase letters 
 indicate lack of statistically significant difference ( analysis by column ) . 
 p<0.05 box plots : a ) results after 7 days in water storage ; b ) results after 3 months in 
 water storage
the fact that there were no differences among thirds and faces is probably due to the 
 ceramic thickness employed ( 1.5 mm on axial walls and 2.0 mm on the occlusal aspect ) and 
 the composition of ceramic ips e.max press , a vitreous ceramic composed of 
 lithium disilicate , which may have 
 allowed sufficient light transmission throughout the crown extent .
( 2008 ) also found uniform 
 microhardness values along the cement layer for the cement variolink ii used to cement 
 leucite - based ceramic crowns with 1.4 mm and 2.0 mm of thickness . concerning the type of substrate , only the composite resin core , after storage in water 
 for 7 days , resulted in significantly lower hardness values when compared with the other 
 groups and storage conditions .
when full crowns are cemented on metal or fiber - reinforced resin posts or cores , the 
 permeability of the simplified adhesive will not be in effect .
but when the substrate is 
 hydrated dentin , the permeability might be more relevant and harmful than the chemical 
 incompatibility in relation to metal and composite resin substrates .
this occurs because the fluid 
 transudation through the adhesive may result in water accumulation at the interface 
 between adhesive and cement , causing significant reductions in bond strength .
this water 
 accumulation is originated from the hydrated dentin , and the negative effect of this 
 permeability on the adhesive resistance of resin cements was confirmed by in 
 vitro studies .
when 
 crowns are cemented on core substrates other than dentin , the permeability is absent or 
 reduced , but the chemical incompatibility persists between the acidity of simplified 
 adhesives and the components of the chemical curing route of resins .
the reduction of mechanical properties 
 of composites stored in water is predominantly related to water absorption by the 
 polymer , which is softened by the tumescence of polymeric chains and reduction of 
 frictional strength of these chains .
once saturated in water , the polymeric 
 chains are stabilized and there is no further reduction of material properties .
the effects of humidity on the 
 mechanical properties of resin cements have been extensively investigated , and there is 
 consensus that the action of solvents on the polymeric chain is deleterious to the 
 mechanical properties of the cement . as previously mentioned , it has been reported that resin monomers originated from 
 two - step conventional and one - step self - etching adhesives may impair the co - curing and 
 consequent bond between these types of adhesives and composites , whose curing reaction 
 is initiated by a redox reaction between the tertiary amine and benzoyl peroxide . as a 
 consequence ,
low adhesive strength values are reported when these materials are 
 combined . in an attempt to avoid this chemical incompatibility 
 and enhance the adhesive strength ,
manufacturers have been adding co - initiators in 
 adhesive systems that react with acidic resin monomers and produce phenyl or 
 benzenesulphonic radicals that initiate the curing reaction in dual resin cements or 
 when there is no adequate light exposure .
the microbrush of the adhesive system used in this study contains initiators that are 
 fundamental for the self - cure mechanism .
additionally , this mechanism requires other 
 initiators that are originated from a composite that also presents a self - cure 
 mechanism and may come from a dual or chemically cured 
 reconstruction composite ( when the adhesive is used for reconstruction ) or dual or 
 chemically cured cement ( in cases of luting ) .
that is to say , the self - cure mechanism of 
 dsc adhesive may not occur if it does not get in contact with dual or 
 self - cured resin .
for this reason , if the dsc adhesive is exclusively 
 used with a light - cured resin , it should necessarily be light - cured before placement of 
 composite resin . based on this assumption , it is understood that there clearly is a chemical reaction 
 between the dsc adhesive and self - cured or dual - cured resins .
this reaction should favor 
 both the adhesive and cement curing . in this context ,
the dsc adhesive layer applied on 
 the substrates , though light - cured , contained initiators of chemical reaction ready to 
 react with initiators present in the cement variolink ii and provide fast 
 consolidation of curing of the dsc adhesive , also favoring the cement curing , especially 
 in areas less accessible to light , thus characterizing a " collaboration " reaction of 
 initiators of the adhesive and cement to enhance the curing of both . considering that 
 the quantity of such initiators in the adhesive is limited to the applied layer , when 
 the adhesive was applied on dentin , all radicals were free to react with the cement . 

however , when the adhesive was applied on the core resin , it is speculated that some 
 initiators of the adhesive reacted with uncured free radicals of the core resin and 
 consequently reduced the availability of initiators to react with the cement .
the 
 consequence was that the cement cure and certainly also the adhesive cure were delayed , 
 resulting in lower microhardness values in the initial storage period ( 7 days ) .
analysis 
 of table 1 reveals that , although not 
 statistically significant , the microhardness of group r after 7 days of storage 
 decreased from the cervical margin to the occlusal aspect , suggesting that in areas 
 close to margins , in which the light acts on the cement curing with greater intensity , 
 the conflict of utilization of initiators between core resin and cement is surpassed by 
 the curing achieved by light - curing .
since the effects of this conflict are temporary , 
 causing only a delay in the curing process , the same phenomenon is not observed after 3 
 months of storage .
the composite resin , when used as filling material for cores , provides advantages as 
 easy handling , fast curing , good translucency shade , which does not interfere with the 
 ceramic shade .
however , in the routine clinical practice , the resin is often in contact 
 with saliva for a considerable time . it is not known to which extent the lower hardness 
 outcomes observed when cementing over the composite resin core may cause any relevant 
 clinical problem .
further studies should be conducted to enhance the understanding on 
 the reactions occurring between this substrate and the resin cement .
based on the results , the following could be concluded : there was no significant difference in the microhardness results between the cervical , 
 medium , and occlusal thirds and occlusal surface ; there was significant difference in the microhardness results between substrates . for 
 the 7-day storage period , the results of the composite resin substrate were lower than 
 dentin and metal .
after 3 months of storage , the results were similar for the 3 
 substrates ; the type of material employed for coronal reconstruction of preparations for prosthetic 
 purposes may influence the cement properties .
 composite resin and metallic posts are the materials most employed for reconstruction 
 of teeth presenting partial or total destruction of crowns . 
 resin - based cements have 
 been widely used for cementation of ceramic crowns . 
 the success of cementation 
 depends on the achievement of adequate cement curing.objectivesto evaluate the microhardness of variolink ii ( ivoclar vivadent , 
 schaan , liechtenstein ) , used for cementing ceramic crowns onto three different 
 coronal substrate preparations ( dentin , metal , and composite resin ) , after 7 days 
 and 3 months of water storage . 
 the evaluation was performed along the cement line 
 in the cervical , medium and occlusal thirds on the buccal and lingual aspects , and 
 on the occlusal surface . 
 material and methodsthirty molars were distributed in three groups ( n=10 ) according to the type of 
 coronal substrate : group d- the prepared surfaces were kept in dentin ; groups m 
 ( metal ) and r ( resin)- the crowns were sectioned at the level of the cementoenamel 
 junction and restored with metallic cast posts or resin build - up cores , 
 respectively . 
 the crowns were fabricated in ceramic ips e.max press 
 ( ivoclar vivadent , schaan , liechtenstein ) and luted with variolink ii . 
 after 7 
 days of water storage , 5 specimens of each group were sectioned in buccolingual 
 direction for microhardness measurements . 
 the other specimens ( n=5 ) were kept 
 stored in deionized water at 37c for three months , followed by sectioning and 
 microhardness measurements . 
 resultsdata were first analyzed by three - way anova that did not reveal significant 
 differences between thirds and occlusal surface ( p=0.231 ) . 
 two - way anova showed 
 significant effect of substrates ( p<0.001 ) and the tukey test revealed that 
 microhardness was significantly lower when crowns were cemented on resin cores and 
 tested after 7 days of water storage ( p=0.007 ) . 
 conclusionthe type of material employed for coronal reconstruction of preparations for 
 prosthetic purposes may influence the cement properties . 