a evoluo dos aparelhos fotopolimerizadores pode ser notada nos diferentes sistemas introduzidos recentemente no mercado .
a tecnologia apresentada pelos aparelhos led promete maior tempo de vida til , no gerar aquecimento e produzir luz especfica para a ativao da canforoquinona .
no entanto , ainda so necessrios estudos complementares para se conhecer a real efetividade destes aparelhos na polimerizao dos materiais .
neste trabalho foi verificada a microdureza de 4 cores ( b-0,5 , b-1 , b-2 e b-3 ) da resina composta filtek z-250 ( 3 m espe ) quando polimerizadas com 4 fontes de luz , sendo uma halgena ( ultralux - dabi atlante ) e trs led ( ultraled - dabi atlante , ultrablue - dmc e elipar freelight  3 m espe ) .
os 192 corpos - de - prova foram distribudos em 16 grupos e os materiais foram inseridos em nico incremento em matrizes cilndricas de 4 mm x 4 mm , sendo polimerizados pelo tempo preconizado pelo fabricante .
em seguida , foram submetidos ao teste de microdureza na superfcie superior e inferior dos cilindros .
os valores de dureza obtidos foram submetidos  anlise de varincia e ao teste de tukey ao nvel de 5% .
foi observado que a dureza dos corpos - de - prova variou conforme a cor do material e aparelhos utilizados .
o aparelho led que emite maior intensidade luminosa proporcionou a obteno dos maiores valores de dureza , com o croma b-0,5 possibilitando a melhor polimerizao .
tambm foi observado que a regio do fundo dos corpos - de - prova foi mais sensvel  mudana das cores .
a intensidade de luz dos fotopolimerizadores led  fundamental para seu bom funcionamento , principalmente quando empregadas resinas com croma mais acentuado .
the use of composite resins has appeared in dentistry as a response to the expectations to achieve a material that might fulfill the requirements of the oral environment and also allow achievement of restorations with proper function and especially esthetics19 .
light cured composite resins offer several advantages compared to early restorative systems , and their curing depends on application of light on their surface11,27 .
curing depth is often considered a primary factor for clinical success of composite resin restorations , since it directly affects the physical properties of materials and longevity of restorations .
the factors that may affect the curing of resin materials according to the literature7,17,25,28 include those directly related to the restorative material , including composite resin shade , amount of photoinitiators , organic and filler matrixes .
similarly , light curing units also play a fundamental role for proper curing , especially concerning the period of exposure and general status of the equipment .
therefore , in an attempt to optimize the utilization of light cured composites , manufacturers of light curing units have developed different curing systems , including those with soft start mechanism , which gradually increase light intensity , and those that emit a constant light intensity higher than 1,000mw / cm .
later , light emitting diode ( led ) light curing units were developed , which apply a lower light intensity than conventional light curing units , yet are able to cure resin materials , because of light emission in specific wavelength for activation of camphorquinone ( 450 to 480nm)10,14 .
however , pires , et al.21 ( 1993 ) , employing halogen light curing units , observed a strong correlation between light intensity and microhardness , revealing a small depth of curing or reduction in microhardness values when the light intensity applied to the composite was reduced .
however , more recent studies ( price et.al . , 2004)24 revealed that when the energy density ( j / cm ) is maintained , the increase in intensity is no related with the curing depth .
kawaguchi14 ( 1994 ) investigated halogen light curing units following guidelines of the international organization standardization ( iso ) dis 4049 , and highlighted that the coefficient of light transmission of different materials also influences their curing , being that some materials may be cured in depth with lower light intensity than others . within this context , the factors that may affect the curing of materials include those related to the restorative material , including the resin shade , amount of photoinitiators , organic and inorganic matrix ; the operator , including the distance and orientation of light beams and restorative technique ; and types of light curing units , concerning the emission spectrum and association between light intensity , period of exposure and general status of the equipment1,7,11 . based on these data , doubts have arisen : are led curing units effective ? what is the performance of these units to cure different resin shades ?
all these factors and doubts indicate the importance to investigate the real curing ability of different light curing systems available in dental market
. moreover , composite resins currently available are highly complex and technically sophisticated materials . therefore , to avoid selection of a certain product by the subjective preference of the operator , it is important to understand its physical and mechanical properties , as well as its performance after utilization of different light curing units17 .
therefore , the present study evaluated the microhardness of different composite resin shades , after utilization of a halogen light source and three light emitting diode ( led ) curing units , considering readings performed on the surface close to the light source and also at 4 mm in depth . led units emitting similar wavelengths with different power densities were compared with a conventional halogen light unit that emits a wider light spectrum .
the null hypothesis to be tested was that either the shade composite resins or curing equipment would not interfere with the microhardness values .
a total of 192 specimens were fabricated , which were distributed into 16 study groups .
specimens were fabricated with aid of acrylic cylindrical templates with a central perforation measuring 4 mm in diameter and 4 mm in height . for fabrication of specimens
, templates were positioned on a glass slab and completely filled in a single increment , with some excess , with the composite resin filtek z-250 ( 3 m espe ) of different shades ( b-0.5 , b-1 , b-2 and b-3 ) .
then , a polyester strip ( 3 m espe ) and another glass slab were positioned on composite resin , with application of force against the composite resin template .
after 10 seconds , the top glass slab was removed , the polyester strip was kept in position and each specimen was submitted to the light emitted by the light curing units for 20 seconds , as recommended by the manufacturers .
specimens fabricated with the b-0.5 shade were light cured for 30 seconds , as recommended by the manufacturer for this shade .
light curing units employed were the ultraled unit ( dabi - atlante ) , with 7 leds and light intensity of 200mw / cm ; ultrablue i unit ( dmc ) , with 7 leds and 140mw / cm ; elipar freelight unit ( 3 m 
espe ) , with 19 leds and 400mw / cm ; and ultralux halogen light unit ( dabi atlante ) with 380mw / cm .
all units were measured with radiometer cure demetron and were connected to a voltage stabilizer revolution ii ( sms ) to allow maintenance of their power .
then , templates filled with composite resin were labeled and marked on the top surface for identification of the region closest to the light source .
thereafter , specimens were cleaned with a detergent solution in ultrasound , branson 2210 , for 10 minutes , to remove debris occasionally accumulated during handling of specimens . for achievement of a surface with no staining
surface microhardness of specimens was measured with hmu-2000 shimadzu , with utilization of a knoop indenter of 20 g for 5s . this way , 3 indents were performed on top surface and further 3 on bottom surface , which was more distant from the light source .
considering that each of the 16 study groups had 12 specimens that received 3 indents , original data comprised 576 microhardness values for each surface .
statistical analysis was performed by calculation of mean values of 3 measurements performed for each surface , adding up to 192 independent data analyzed for the top surfaces and 192 for the bottom surfaces .
results were submitted to analysis of variance ; whenever a significant difference was found , tukey test at the 5% level was applied .

 table 1 presents original microhardness means and standard deviation of each study group with regard to the top and bottom aspects of specimens .
analysis of both surfaces with regard to different light curing units indicated a statistically significant difference for all study groups ; elipar freelight unit ( 3m - espe ) provided highest values of surface hardness , regardless of the composite shade .
the lowest values were achieved with the ultrablue unit ( dmc ) , whereas the ultralux ( dabi atlante ) and ultraled units ( dabi atlante ) , respectively , presented intermediate results ( tables 2 ) .
dms ( top ) = 0.0235 dms ( bottom)= 0.0242 however , when the composite resin shades were considered at the top surfaces , groups of b-0.5 shade presented highest hardness values , whereas other shades presented similar results . on the other hand , for the bottom surfaces , all had a significant effect on the efficacy of the units .
therefore , groups of specimens of b-0.5 shade presented the highest means , followed by shades b-1 , b-2 and b-3 , respectively ( table 3 ) .
currently , there has been considerable emphasis on the concentration of photoinitiators and wavelength employed to cure resin materials , since they are in charge of conversion of monomers into resin polymers , which determine the curing of resin material .
it is known that concentration of camphorquinone varies between materials and may be altered according to shade and translucency of the material6 .
thus , curing of resin materials has been subject of several studies with different methodologies 4,5,20 .
these include microhardness test , which is one of the most often employed methods and is able to provide reliable results and reproducibility to the study . according to shortall and harrington26 ( 1996 ) , intrinsic characteristics of each material , as composition , hue and shade ,
may interfere with the curing depth ; an appliance able to transmit adequate light intensity with correct wavelength is required to provide good results11 . with regard to the appliances analyzed in the present study , halogen light unit is the most widely employed and , because it works in a wider light spectrum , besides curing the restorative material , it may also heat the tooth and composite resin during the process23 .
these appliances usually work in wavelengths of 400 nm to 700 nm ; however , only wavelength nearly 470 nm is strongly absorbed by photoinitiator ( camphorquinone)23 .
period of exposure also plays an important role in the curing process , since the surface hardness of composite resin is significantly increased when exposed for longer periods10,15,26 .
moreover , caughman , et al.5 ( 1995 ) and feilzer , et al.7 ( 1995 ) also agree that light intensity , wavelength and time of exposure are critical variables for achievement of maximum curing of composite resins . considering these observations , analysis of microhardness values achieved i.e. on
this may be related to the wavelength emitted by this unit , which according to its manufacturer coincides with the maximum absorption of camphorquinone , explaining its better performance compared to halogen light units .
compared to other led units tested , elipar freelight unit probably was more successful because of the higher light intensity emitted . within this context , according to vieira , et al.34 ( 1999 ) and briso4 ( 2003 ) , light emission in low intensity yields inadequate light curing of composite resins , even if the time recommended by the manufacturers is followed , and may negatively influence physical properties and clinical performance of these materials . similarly ,
ultralux unit presented better performance than ultraled and ultrablue units because of higher light intensity .
the lower mean hardness values found for ultraled and ultrablue units , which are also led units , are assigned to lower light intensity emitted and improper wavelength for curing composite resins employed .
frentzen , et al.9 ( 2001 ) compared a halogen light unit and a led unit and reported that led unit achieved higher hardness values ; however , authors did not mention the light intensity emitted by the units , which impairs comparison with the present results .
studies as that conducted by pereira , et al.20 ( 2001 ) corroborate the present findings , concluding that units emitting higher light intensities have a stronger ability of curing and thereby allow achievement of higher hardness values .
these aspects may also be taken into account for analysis of the ultrablue unit , which was less effective than all other units tested .
bosquiroli3 ( 2003 ) highlighted that resins may have different photoinitiators , and thus the emission spectrum of the equipment may be na important factor for the final properties of restorations . according to the author , the energy required for each composite should be indicated on the label , and similarly the light curing units should indicate the light intensity emitted and corresponding wavelength .
these observations are extremely relevant for led units applied on materials containing camphorquinone as photoinitiator . however , other photoinitiators require energy at other wavelengths and in these cases some units currently available will not effectively cure these materials32 .
when the influence of shade on microhardness is individually considered , the results demonstrate that shade b-0.5 presented higher values than other shades .
this is primarily related to the period of exposure suggested by manufacturer , which is 10 seconds more than recommended for shades b-1 , b-2 and b-3 . according to baharav , et al.1 ( 1997 ) and pereira , et al.19 ( 2000 )
, the time of exposure to light is the most significant aspect influencing higher hardness values and consequently greater curing depth .
there was also significant reduction in microhardness on the bottom aspect of specimens in all study groups .
despite of that , in general the bottom aspect presented the same tendency observed at the top surface , i.e. units with higher light intensities ( elipar freelight and ultralux , respectively ) , were better than the others ( ultraled and ultrablue ) , with a statistically significant difference in the performance of all units .
it should be highlighted that low light intensities may only cure composite superficially , therefore impairing the clinical performance of restorations and giving the false impression that the material is well cured1 .
this information was demonstrated by the observation that , on the bottom surface of specimens , the composite shade influenced the performance of units .
thus , on the bottom surface , shade b-0.5 provided higher hardness values , followed by resins b-1 , b-2 and b-3 , respectively .
this was probably due to the distance from light source , as well as because the curing depth is inversely proportional to hue value .
therefore , darker materials possibly impair light penetration into the deepest regions of specimens , as also observed by tanoue et al.29 ( 2001 ) . considering the means achieved by the groups
, this effect seems to be more significant in specimens submitted to units that emit lower light intensities ( g - viii and g - xii ) .
this was also noticed by ferracane , et al.8 ( 1997 ) , who related the curing depth of different composite resins to shade and translucency of materials , concluding that lighter shades allow greater curing depth .
in fact , the present results are in agreement with the findings of cook and standish6 ( 1983 ) and kanca iii13(1986 ) who stated that pigments employed to provide shades to restorative materials may absorb the light passing through the resin and impair curing .
studies conducted by pollack and lewis22 ( 1981 ) , thirta , et al.30 ( 1982 ) , cook and standish6 ( 1983 ) and swartz , et al.27 ( 1983 ) , also reveal that light cured composite resins of dark shades require longer periods of exposure , and curing in thinner layers is recommended to allow higher hardness values . however , despite the present considerations , it should be highlighted that lohbauer , et al.16 ( 2005 ) tested several photoinitiator equipments and stated that conventional halogen light units allow less loss of mechanical properties of dental composites , even though led units have been presenting encouraging outcomes when employed with compatible resin materials inserted with proper thickness2,12,18,31,33 .
thus , it seems evident that selection of light curing unit should be considered an important step to conduct clinical work with resin materials .
it should also be mentioned that many types of units are introduced in the market every year , whose manufacturers state their proved efficiency , low cost and easy maintenance .
however , professionals should base their decisions in well - designed scientific studies , besides searching for constant update , not being limited to reading folders and technical profiles supplied by manufacturers and/or representatives .
these aspects demonstrate that correct selection should be based on impartial studies , and materials introduced in the market may not have been adequately tested for utilization in the dental clinic .
on the basis of results achieved , the following could be concluded : in both led and halogen light units , the microhardness values ranged according to the shade of resin and appliances employed ; shade was inversely proportional to the surface hardness of resin , with the highest values observed for shade b-0.5 ; led units with low light intensity provided lower hardness values than high intensity units ; aspects distant from the light source were more sensitive to changes in shades .
the emission of light at a specific wavelength for activation of camphorquinone by a unit did not lead to successful curing of the materials investigated .
 introduction : the evolution of light curing units can be noticed by the different systems recently introduced . 
 the technology of led units promises longer lifetime , without heating and with production of specific light for activation of camphorquinone . 
 however , further studies are still required to check the real curing effectiveness of these units.purpose:this study evaluated the microhardness of 4 shades ( b-0.5 , b-1 , b-2 and b-3 ) of composite resin filtek z-250 ( 3 m espe ) after light curing with 4 light sources , being one halogen ( ultralux  dabi atlante ) and three led ( ultraled  dabi atlante , ultrablue  dmc and elipar freelight  3 m espe).methods:192 specimens were distributed into 16 groups , and materials were inserted in a single increment in cylindrical templates measuring 4 mm x 4 mm and light cured as recommended by the manufacturer . 
 then , they were submitted to microhardness test on the top and bottom aspects of the cylinders.results:the hardness values achieved were submitted to analysis of variance and to tukey test at 5% confidence level . 
 it was observed that microhardness of specimens varied according to the shade of the material and light sources employed . 
 the led appliance emitting greater light intensity provided the highest hardness values with shade b-0.5 , allowing the best curing . on the other hand , appliances with low light intensity 
 were the least effective . 
 it was also observed that the bottom of specimens was more sensitive to changes in shade.conclusion:light intensity of led light curing units is fundamental for their good functioning , especially when applied in resins with darker shades . 


Proposta:
Mtodos:
Resultados:
Concluses:
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
CONCLUSIONS