light - cured composite resins have been considered a material of major importance in restorative dentistry due to their esthetic properties .
however , achievement of clinical and esthetic success with this material involves several aspects , including the curing technique .
the process of curing of composite resins occurs in three main phases : pre - gel , gel point and post - gel . during the pre - gel phase ,
the material may flow and undergo molecular rearrangement , in order to compensate shrinkages forces . during this phase
thereafter , the resin passes from the flow state ( pre - gel ) to the viscous state ( post - gel ) , which establishes its gel point . during the post - gel phase
, the resin presents a high modulus of elasticity , loses its flowing ability and transmits the stress yielded by polymerization shrinkage to the tooth - restoration interface5,15,16 . at this stage , there is a predominance of cross links in the polymer structure .
studies have demonstrated that curing technique may influence the polymerization shrinkage of composite3,4,8 - 11,15 .
the incomplete curing of composite resins is associated to a reduction in their mechanical properties and biocompatibility , increased content of residual monomers and altered clinical performance due to esthetic impairment , with high tendency to surface staining and possibility of marginal leakage .
therefore , different curing techniques have been suggested to overcome the problems related to polymerization shrinkage , especially postoperative sensitivity and marginal leakage3 - 7 : stepped technique ( 2 stages )  low light intensity is applied for a determined period , followed by high light intensity for a certain additional period .
ramped technique ( progressive )  initial low light intensity is applied , which is gradually increased for a certain period until it reaches a high final value that s maintained until completion of exposure .
pulsed - delay technique ( delayed pulse )  initial low light intensity is applied for a certain period , which is sufficient to allow curing of the surface .
surface finishing and polishing should be performed during this period , followed by a second exposure at a higher light intensity .
the aim of these techniques , which comprise initiation of light - curing at a low light intensity and a delay time , is to allow the occurrence of a more evident pre - gel phase , which would provide a low rate of conversion of monomers and thus allow material flow , yielding low internal stress from shrinkage and providing a good marginal adaptation . at the final stage of these techniques ,
completion of curing at a high light intensity would provide a proper degree of conversion , which is required for the achievement of satisfactory physical and mechanical properties4,6,7,10,11,18 .
the measurement of microhardness is an indicator of the mechanical , physical and biological properties of a restorative material7 .
application of this test in depth as a parameter for analysis of curing is justified , since studies have indicated a good correlation between the knoop hardness number ( khn = kg / mm ) and infrared light spectroscopy , a direct method that evaluates the degree of conversion of monomers6 . the aim of this study was to evaluate in vitro the effect of four light - curing techniques on the depth of cure of a composite resin by microhardness testing .
the null hypothesis was that there is no difference between different curing techniques among different depths of cure from the composite resin surface . to test this null hypothesis ,
a factorial study 44 was conducted and the factors under study were cure technique at 4 levels : traditional / continuous ( t ) , ramped ( r ) , stepped ( s ) and pulse - delay ( pd ) , and depth of cure at 4 levels 0.1 , 1.0 , 2.0 and 4.0 mm from the lighted surface , resulting in 16 groups .
the specimens were longitudinally sectioned , polished and microhardness measurements were done at established distances from the surface .
the specimens were fabricated in cylindrical cavities ( 3.0 mm of diameter  7.0 mm of height ) prepared in polyvinyl chloride ( pvc ) plates ( figure 1 ) .
ten specimens were fabricated for each light - curing method : traditional ( continuous ) , ramped ( exponential ) , stepped ( two stages ) and pulsed - delay ( delayed pulse ) .
the traditional curing technique was applied with the curing lite 2500 unit ( 3m / espe ) .
the intensity was kept continuous at 600 mw / cm for a constant and continued period of 40 s. the ramped technique is one of the functions of the optilux 501 unit ( demetron , kerr , orange , ca , usa ) . by this function ,
the light intensity applied was exponential , reaching 1000 mw / cm during the 10 initial s , followed by additional 10 s at this intensity .
the stepped technique ( s ) was applied with the vip unit ( bisco inc . ,
the light intensity employed for this technique was followed by two stages , being the first at low intensity and the last at a high light intensity .
the values of this intensity were 200 mw / cm for 10 s and 600 mw / cm for 30 s , respectively .
the pulsed - delay technique comprised treatment at high and low light intensities with a time interval .
the intensities applied were 100 mw / cm for 5 s , followed by a time interval of 3 min and a final light - curing at 600 mw / cm for 30 s. after curing , the pvc plates were longitudinally sectioned throughout the specimens with diamond discs ( diamond wafering blade , 15hc # 11 - 4244 ; buehler ltd . , lake bluff , il , usa ) coupled in a saw (
the cut surfaces of the specimens were finished and polished in a polishing machine ( arotec apl 4 ; arotec ind .
so paulo , sp , brazil ) , with serial silicon carbide papers of decreasing abrasiveness ( grits 320 , 400 , 600 and 1200 ) and felt ( 6 m , 3 m , 1 m ) with a diamond suspension ( buehler metadi ; buehler ltd . ) with grits corresponding to felts .
the specimens were stored under moist conditions at  37c for 24 h before the microhardness determination .
depth of cure was determined by a microhardness tester ( future tech with software fm - ars ) with a 25 g load for 5 s. this device provided the knopp hardness ( kg / mm ) .
for each half of specimen was made three indentations , resulting in six indentations per specimen .
they were made at the depths of 0.1 , 1.0 , 2.0 and 4.0 mm from the surface where the polymerization was made ( figure 1 ) .
the six indentations were averaged ( n=10 for each depth ) . the ratio of hardness at 1.0 , 2.0 and 4.0 mm from the surface related to the value at 0.1 mm was calculated and the value 0.80 ( 80% ) is considered acceptable .
the variables under study were cure technique at 4 levels and depth in the specimen at 4 levels with 10 repetitions , resulting in a sample number of 4  4  10 .
data were subjected to two - way anova at the significance level of 5% , for observation of the relationship between microhardness , depth and interaction between the light - curing methods investigated , followed by tukey test .
, cary , nc , usa ) was used for the analysis and the significance level set at 5% .
the null hypothesis was that there is no difference between different curing techniques among different depths of cure from the composite resin surface . to test this null hypothesis ,
a factorial study 44 was conducted and the factors under study were cure technique at 4 levels : traditional / continuous ( t ) , ramped ( r ) , stepped ( s ) and pulse - delay ( pd ) , and depth of cure at 4 levels 0.1 , 1.0 , 2.0 and 4.0 mm from the lighted surface , resulting in 16 groups .
the specimens were longitudinally sectioned , polished and microhardness measurements were done at established distances from the surface .
the specimens were fabricated in cylindrical cavities ( 3.0 mm of diameter  7.0 mm of height ) prepared in polyvinyl chloride ( pvc ) plates ( figure 1 ) .
ten specimens were fabricated for each light - curing method : traditional ( continuous ) , ramped ( exponential ) , stepped ( two stages ) and pulsed - delay ( delayed pulse ) .
the traditional curing technique was applied with the curing lite 2500 unit ( 3m / espe ) .
the intensity was kept continuous at 600 mw / cm for a constant and continued period of 40 s. the ramped technique is one of the functions of the optilux 501 unit ( demetron , kerr , orange , ca , usa ) . by this function , the light intensity applied was exponential , reaching 1000 mw / cm during the 10 initial s , followed by additional 10 s at this intensity . the stepped technique ( s )
the light intensity employed for this technique was followed by two stages , being the first at low intensity and the last at a high light intensity .
the values of this intensity were 200 mw / cm for 10 s and 600 mw / cm for 30 s , respectively .
the pulsed - delay technique comprised treatment at high and low light intensities with a time interval .
the intensities applied were 100 mw / cm for 5 s , followed by a time interval of 3 min and a final light - curing at 600 mw / cm for 30 s. after curing , the pvc plates were longitudinally sectioned throughout the specimens with diamond discs ( diamond wafering blade , 15hc # 11 - 4244 ; buehler ltd . , lake bluff , il , usa ) coupled in a saw (
the cut surfaces of the specimens were finished and polished in a polishing machine ( arotec apl 4 ; arotec ind .
so paulo , sp , brazil ) , with serial silicon carbide papers of decreasing abrasiveness ( grits 320 , 400 , 600 and 1200 ) and felt ( 6 m , 3 m , 1 m ) with a diamond suspension ( buehler metadi ; buehler ltd . ) with grits corresponding to felts .
the specimens were stored under moist conditions at  37c for 24 h before the microhardness determination .
depth of cure was determined by a microhardness tester ( future tech with software fm - ars ) with a 25 g load for 5 s. this device provided the knopp hardness ( kg / mm ) .
for each half of specimen was made three indentations , resulting in six indentations per specimen .
they were made at the depths of 0.1 , 1.0 , 2.0 and 4.0 mm from the surface where the polymerization was made ( figure 1 ) .
the six indentations were averaged ( n=10 for each depth ) . the ratio of hardness at 1.0 , 2.0 and 4.0 mm from the surface related to the value at 0.1 mm was calculated and the value 0.80 ( 80% ) is considered acceptable .
the variables under study were cure technique at 4 levels and depth in the specimen at 4 levels with 10 repetitions , resulting in a sample number of 4  4  10 .
data were subjected to two - way anova at the significance level of 5% , for observation of the relationship between microhardness , depth and interaction between the light - curing methods investigated , followed by tukey test . the sas program ( version 8.02 , sas institute inc .
, cary , nc , usa ) was used for the analysis and the significance level set at 5% .
knoop microhardness mean values recorded for the specimens using the four study methods , as well as the standard deviation and percent curing in relation to the most superficial region are presented in table 1 . up to the depth of 2.0 mm
, all curing techniques showed an acceptable relative hardness compared to the 0.1 mm value ( 0.80 ) ( table 1 ) .
different letters indicate statistically significant differences ( p<0.05 ) ; uppercase letters compare the effect of depth of cure for each curing method ( within columns ) and lowercase letters compare curing methods for each depth of cure ( within lanes ) .
[ microhardness ratio relative to the 0.1 mm value ] there was a statistically significant effect for the interaction method of cure distance from the surface and for the isolated factors ( p<0.05 ) .
for all depths of cure , the traditional method provided higher knoop microhardness values compared to the new light - curing methods , stepped and pulsed ( p<0.05 ) .
the ramped method was similar to the traditional method at depths of 0.1 and 1.0 mm ( p>0.05 ) but lower values at the deepest depth ( p<0.05 ) .
the microhardness values decreased according to the distance from the external surface ( p<0.05 ) , irrespective of the curing method used .
ideally , the degree of curing should be the same throughout the depth of the restoration , and the ratio of hardness ( more external / more internal ) should be equal to 1 or a close value .
when a composite resin increment is cured , light passes through its interior and loses intensity due to dispersion , leading to a lower curing effectiveness14 .
this light dispersion inside the bulk of material leads to the difference in microhardness between the external and internal surfaces .
light - curing of composite resins is considered adequate when this proportion is equal to or higher than 80% ( n=0.80)7,14 . with regard to the mean knoop microhardness values in depth and the methods evaluated , the ratio of curing was higher than that reported by those authors up to the depth of 2.0 mm ( table 1 ) .
these data confirm the recommendation that resin increments should be limited to a maximum thickness of 2.0 mm for achievement of satisfactory curing3,4,6,7,10,14,15 . concerning the effectiveness of curing of the new modulated techniques
, the results of the present study indicated that they are favorable for curing , as demonstrated by the knoop microhardness test , when these techniques were applied using the same curing times , intensities and materials investigated , according to other studies3,4,7,15 - 18 . regarding the new light - curing techniques , the ramped method was able to cure 2.0 mm of composite resin in a shorter time ( 20 s ) , even at a lower light density , when compared to the traditional method .
considering that the higher the degree of conversion , the higher the microhardness , the present outcomes confirm those of bouschlicher , et al.3 .
similarly to the present microhardness values for the ramped and traditional techniques up to 2 mm in depth , those authors found a similar degree of conversion between these two methods , which was able to reduce the conversion rate up to 1 mm below the most external surface by the ramped method .
however , this did not significantly affect the total conversion of the material with use of both methods .
initially , the lower conversion rate with the ramped method allowed a reduction in the stress rate and maximum shrinkage stress , with no damage to the physical properties of the restorative material6,12,13,15,17 .
other studies investigated the forces yielded during polymerization shrinkage and demonstrated that the pre - gel phase is extremely fast in composite resins cured by high light intensities as used in conventional continuous ( 600mw / cm - 40 s ) , which therefore is a negative aspect of this technique8,10,12,17 .
the microhardness values achieved by the stepped and pulsed methods were lower than those obtained with the traditional method . despite the numerical difference between groups
as seen in table 1 , those methods provided satisfactory curing , as demonstrated by de wald , et al.7 . and rueggeberg , et al.14 . in order to validate the mechanical behavior of a composite resin and , probably from a clinical point of view
, it could not be consider as an isolated element to affect the behavior of the restoration .
with regard to the stepped technique , the results found by bouschlincher , et al.2 , comprising comparison of the force intensity and maximum shrinkage force , did not demonstrate any difference between the traditional and stepped methods . despite the initial low light intensity ,
curing techniques may give rise to reduce polymerization shrinkage due to stress relief by allowing flow to occur during setting3,4,8 , some material 's chemical and physical properties could be negatively affected , producing a polymer mechanically more fragile .
a recent study2 found that low power density applied to the composite resin at the initial periods of polymerization reaction resulted in polymers with increased susceptibility to softening in ethanol , in spite of achieving a comparable degree of cure to that of the conventional continuous technique .
it has been hypothesized that slow start polymerization are probably associated with relatively few centers of polymer growth , generating a small number of free radicals , resulting in a more linear polymeric structure , with lower cross linking density , as evidenced by reduced glass transition temperature and increased susceptibility to ethanol degradation2 .
although the pulsed technique yielded lower microhardness values than those achieved by the traditional method , due to the lower light density , the material also presented an acceptable degree of curing ( n=0.80)7,14 . the outcomes of other studies7,10,14 - 18 corroborate this finding , with the report that initial low light intensity does not lead to a considerable decrease in the mechanical properties .
moreover , specific literature has demonstrated that light - curing techniques comprising initial low light intensity provided better marginal sealing than those that use high light intensity1 - 6,9 - 12 .
further clinical and laboratory studies on the force and shrinkage vectors and the gaps yielded by the characteristic inherent to this material should be conducted , in order to determine the real performance of curing techniques in relation to shrinkage and performance of composite resin restorations in a long - term basis .
composite resin curing by the 4 light curing methods provided decreasing means of microhardness in relation to the surface , according to the increase in depths analyzed ; 2 .
at the depth corresponding to the resin increment recommended by the manufacturer ( 2.0 mm ) , all curing techniques provided satisfactory composite hardness ; 3 .
the use of initial low light intensity for curing composite resins does not compromise the microhardness up to 2.0 mm depth .
 the light - curing technique is relevant to reduce the degree of polymerization shrinkage , improving clinical and esthetic success of composite resin restorations.objective:to evaluate in vitro the effect of four light - curing techniques on depth of cure of a composite resin.material and methods : ten specimens of a composite resin were made in cylindrical cavities prepared in pvc plates ( 3.0 x 7.0 mm ) for each light - curing technique . 
 four photoactivation methods were investigated : stepped , ramped , pulse - delay and traditional . 
 specimens were longitudinally sectioned and polished for microhardness measurements ( kg / mm2 ) , which were made at 0.1 , 1.0 , 2.0 and 4.0 mm from the irradiated surface . 
 data were subjected to anova and tukey 's test.results:the effect of factors studied ( curing method and distance from the surface ) and the interaction of these factors was statistically significant ( p<0.05 ) . 
 the traditional method of cure provided higher microhardness values ( 69.6  2.5 ) than the stepped ( 63.5  3.1 ) and pulsed ( 63.9  3.2 ) methods at all depths evaluated , but it did not differ from the ramped method ( 66.7 4.4 ) at 0.1 and 1.0 mm of depth.conclusion:all techniques employed provided satisfactory cure of the composite resin up to the depth of 2.0 mm from the irradiated surface . 