fiber - reinforced composite ( frc ) posts and cores have been proposed as options or substitutes for cast posts and cores in dentistry . unlike the latter , which may be cemented with nonadhesive conventional cements , frc posts should be cemented adhesively .
the clinical performance of frc posts depends substantially on adhesion of the cement to both the root dentin and the post
. however , certain factors may affect the retention of posts , such as ( 1 ) high cavity configuration factors in root canals ( high polymerization stress of resin cements ) , ( 2 ) difficult photopolymerization inside the root canal , and ( 3 ) chemical incompatibility between the adhesive system and the self- or dual - polymerized resin cement .
chemical , dual- , or only photopolymerized adhesive systems and resin cements have been proposed for the cementation of frc posts . in the case of photopolymerized adhesives and cements
the use of translucent quartz- or glass - frc posts would at least theoretically allow transmission of blue light to the apical regions .
however , it has been recently reported that translucent frc posts do not contribute much to polymerization in the middle or apical regions of the root canal .
the literature reports a degree of conversion of 5080% for dental composites . in photoactivated resins
, the degree of conversion varies inside the mass of the material , partly because of dependence on light energy for activation .
some studies showed a significant decrease in the polymerization potential of composites in the intraradicular environment as a result of less light attenuation .
the degree of conversion achieved by a composite provides valid information about the durability and biological safety of the restoration because it affects the mechanical properties and degradation by water and oral acids .
the release of uncured residual monomers presents a potentially sensitizing and irritating factor for the oral tissues .
lack of adequate polymerization , principally in the apical region , may also lead to elution of monomers to the apical foramen .
the objective of this study was , thus , to evaluate the effect of photopolymerization on vickers microhardness of dual - polymerized resin cement at three locations of the intraradicular environment : the cervical , middle , and apical parts of the root canal .
the null hypotheses tested were that ( a ) cervical parts of the roots would show reduced microhardness and ( b ) photopolymerization would increase the microhardness .
soft tissue was cleaned off single - rooted bovine teeth ( n = 10 ) using periodontal curettes .
the coronal and root portions of the teeth were shortened under water - cooling to standardize the length of the specimens at 16 mm .
the root canals were sequentially instrumented ( profile orifice shapers system , dentsply maillefer , ballaigues , switzerland ) and irrigated with 0.5% sodium hypochlorite .
root preparation was performed with a slow - speed calibrated drill ( size no : 3 ) of a two - stage , parallel - sided quartz - frc post system ( light - post , bisco , schaumburg , il ; apical : 1.4 mm ; middle - coronal : 2.2 mm ; length : 20 mm ; fiber type : 70 wt% preimpregnated unidirectional quartz fibers in 30 wt% epoxy resin ; fibers  12 m ) .
they were then randomly divided into two groups according to whether or not they underwent photopolymerization .
all roots received two coats of black nail varnish to ensure that conversion of the cement was only due to light transmitted through the posts .
after preparation of the root canals , 3 mm of the root portion was embedded in a pvc cylinder ( height : 5 mm ; diameter : 7 mm ) using chemically polymerized acrylic resin ( dencrilay ) to facilitate immobility of the root during post cementation .
the following procedures were performed during embedding : ( a ) grooves were prepared on the external apical portion of the specimens with a diamond bur ( no .
3195 , kg sorensen , sp , brazil ) to provide mechanical retention of the specimens within the acrylic resin ; ( b ) the preparation bur of the post system was placed inside the prepared root canal ; ( c ) this assembly was attached to an adapted surveyor so that the long axes of the bur , specimen , and cylinder were parallel to one another and to the y - axis ; and ( d ) the acrylic resin and its monomer ( autoplast , candulor ag , altsttten , switzerland ) were mixed and poured into the pvc cylinder . a multiple - bottle , total - etch adhesive system ( all - bond 2 , bisco ) was applied to the root dentin following the manufacturer 's protocol when cementing the frc posts . after adhesive application using a microbrush ( cavi - tip , svenska dental instrument ab , vrby , sweden ) , excess was removed using paper points .
the specimens were then randomly divided into two groups ( n = 10 , 5 per group ) : group 1 : the posts were cemented with the dual cement ( duo - link , bisco ) and not polymerized .
group 2 : the posts were cemented with the same cement as in group 1 , and the cement was photopolymerized for 40 s ( optilight plus , gnatus ; light intensity : 500 mw / cm ) by applying the light from the most superior part of the post . to condition the frc posts , frc posts of size 2 ( apical : 1.4 mm ;
middle - coronal : 2.2 mm ) were first etched using 37% h3po4 for 1 min , rinsed and dried , and then cemented with the dual cement ( duo - link , bisco ) .
after cementation , the specimens were stored in distilled water at 37c for 24 h. the specimens were fixed to a metallic base in a cutting machine ( labcut 1010 , extec corp . , london , england ) and sectioned perpendicular to the long axis of the root with a diamond disc under water - cooling .
then , six disc - shaped slices of approximately 1.5-mm thick ( nroot region factor : 20 ; nlight cure factor : 30 ) were obtained ( two cervical , two middle , and two apical ) for the regional microhardness evaluation .
the specimens surfaces were finished and polished with silicone carbide papers in sequence ( grit nos .
600 , 800 , 1200 , 1400 , and 2000 ) for 60 s each in a polishing machine at 300 rpm under running water .
the specimens were then analyzed for vickers hardness ( fm-700 , future - tech , equilam , diadema , sp , brazil ) .
the specimens were submitted to a 50-g load for 30 s , and three readings were obtained from three different regions of each specimen .
the mean microhardness ( kg / mm ) of each specimen was obtained from these three readings .
microhardness ( kg / mm ) data were submitted to two - way analysis of variance ( two - way anova ) and repeated measures with microhardness as the dependent variable and polymerization status ( two levels : with and without ) and root region ( three levels : cervical , middle , and apical ) as independent variables .
p values of < 0.05 were considered to indicate statistical significance in all tests [ table 1 ] .
the specimens were fixed to a metallic base in a cutting machine ( labcut 1010 , extec corp . , london , england ) and sectioned perpendicular to the long axis of the root with a diamond disc under water - cooling .
then , six disc - shaped slices of approximately 1.5-mm thick ( nroot region factor : 20 ; nlight cure factor : 30 ) were obtained ( two cervical , two middle , and two apical ) for the regional microhardness evaluation .
the specimens were fixed to a metallic base in a cutting machine ( labcut 1010 , extec corp . , london , england ) and sectioned perpendicular to the long axis of the root with a diamond disc under water - cooling .
then , six disc - shaped slices of approximately 1.5-mm thick ( nroot region factor : 20 ; nlight cure factor : 30 ) were obtained ( two cervical , two middle , and two apical ) for the regional microhardness evaluation .
the specimens surfaces were finished and polished with silicone carbide papers in sequence ( grit nos .
600 , 800 , 1200 , 1400 , and 2000 ) for 60 s each in a polishing machine at 300 rpm under running water .
the specimens were then analyzed for vickers hardness ( fm-700 , future - tech , equilam , diadema , sp , brazil ) .
the specimens were submitted to a 50-g load for 30 s , and three readings were obtained from three different regions of each specimen . the mean microhardness ( kg / mm ) of each specimen was obtained from these three readings .
statistical analyses were conducted using the spss software ( ver . 11.0 for windows ; spss , inc . , chicago , il , usa ) .
microhardness ( kg / mm ) data were submitted to two - way analysis of variance ( two - way anova ) and repeated measures with microhardness as the dependent variable and polymerization status ( two levels : with and without ) and root region ( three levels : cervical , middle , and apical ) as independent variables .
p values of < 0.05 were considered to indicate statistical significance in all tests [ table 1 ] .
photopolymerization did not significantly affect the microhardness values ( p = 0.2239 ; two - way anova ) .
microhardness values also showed no significant difference among the three regions in the root canals ( p = 0.1621 ) .
the mean microhardness values obtained with and without photopolymerization were also not statistically significantly different in the three root regions ( p > 0.05 ; table 2 ) .
mean microhardness values ( kg / mm ) and standard deviations associated with the location in the root canal without ( group 1 ) and with photopolymerization ( group 2 )
dual - cure resin cements were developed with the objective of combining the favorable characteristics of self- and light - activated cements .
these characteristics include effective control of working time and the possibility of achieving adequate degree of conversion even in the absence of light .
other advantages of dual - polymerized cements are their low solubility and superior mechanical and adhesive properties .
in fact , such cements are ideal for situations in which the opacity of the restoration , cavity depth , or root canal may make it difficult for light to reach the full thickness of the cement layer . however , although the two forms of activation of the curing reaction are present , they are complementary and independent . if light exposure is insufficient , the light - activated route of conversion will be affected and maximum polymerization can be compromised .
nevertheless , intraradicular adhesive cementation still presents a significant challenge to clinicians because of the technical variables that are involved and there is little knowledge about the clinical predictability of these materials in the long term . the results of this study indicate that light transmission of the tested translucent quartz frc post was sufficient to achieve similar microhardness of the dual - polymerized resin cement at the cervical , middle , and apical regions of the root canal . thus , the first hypothesis was rejected .
microhardness test results reportedly provided an indirect measurement of the polymerization quality of composite resins .
microhardness test results have also been shown to correlate well with fourier - transform infrared spectroscopy analysis : when hardness values increase , the degree of conversion also increases , and vice versa .
thus , the hardness measurement results obtained in this study indirectly indicate that a higher degree of conversion of dual - cured resin cement may be attained using light - transmitting frc posts . despite relative hardness as an indication of the extent of polymerization
, factors such as load content , type of monomer , concentration of diluents , activation mode , and type and quantity of initiators also play important roles in their respective properties . in the absence of light
, some dual - cured cements reportedly do not reach an adequate degree of conversion .
thus , light - curing is also recommended for dual - cured resin cements . here , the dual - cured cement tested demonstrated microhardness similar to that of the cement tested with and without photopolymerization , resulting in rejection of the second hypothesis too .
generally , impaired adhesion was presented in the studies that dealt with differences in adhesion along the post space .
observed lower microhardness in deeper regions of the root canal , where less light is delivered .
, however , reported that microhardness did not vary significantly along the apical thirds of the roots for panavia and unicem .
they found significantly higher microhardness at the cervical third for both variolink and duo - link , but the mean values were not significantly different between the middle and apical thirds for these cements . on the other hand ,
the results of a recently published study indicate a decrease in vickers hardness from coronal to apical for fiber posts .
future studies should assess whether there is a correlation between adhesion test results and microhardness measurements for testing reliability of dual - polymerized cements in the root canal . against these possible explanations , photopolymerization of the dual - cured cement used in combination with frc posts may be the best practice for the initial polymerization fixation of the post during further chemical polymerization processes .
photopolymerization did not contribute to a significant improvement in microhardness of the cement , and the absence of light did not affect adhesive cementation of the quartz frc post with the cement tested .
 aim : in this study , we evaluated the effect of photopolymerization on vickers microhardness of dual - polymerized resin cement at three locations when a translucent quartz fiber post was used.materials and methods : single - rooted bovine teeth received quartz fiber post systems ( length : 12 mm ) using a dual - polymerized resin cement . in group 1 , the posts were cemented but not photopolymerized , and in group 2 , the posts were both cemented and photopolymerized . 
 after cementation , approximately 1.5-mm thick sections were obtained ( two cervical , two middle , and two apical ) for regional microhardness evaluations.statistical analysis : statistical analyses were performed using the spss software ( ver . 11.0 for windows ; spss , inc . , chicago , il , usa ) . 
 microhardness ( kg / mm2 ) data were submitted to two - way analysis of variance ( two - way anova ) and repeated measures with microhardness values as the dependent variable and polymerization status ( two levels : with and without ) and root region ( three levels : cervical , middle , and apical ) as independent variables . 
 multiple comparisons were made using dunnett 's t3 post - hoc test . 
 p values of < 0.05 were considered to indicate statistical significance in all tests.results:photopolymerization did not significantly change the microhardness values when compared with no photopolymerization . 
 microhardness values also showed no significant difference between the three regions in the root canals in both groups.conclusions:the mode of polymerization of the cement tested in combination with the translucent quartz fiber post system did not affect the microhardness of the cement at the cervical , middle , or apical regions of the root . 
INTRODUCTION
MATERIALS AND METHODS
None
Production of specimens for microhardness measurements
Microhardness measurements
Statistical analyses
RESULTS
DISCUSSION
CONCLUSIONS