improvements in composite mechanical properties have made them more reliable for use in posterior teeth than they were some years ago14,17 . the development of shrinkage during polymerization is the major disadvantage of light cured composites and compomers13,16 
 .the conversion of the monomer molecules into a polymer network is accompanied by a closer packing of the molecules , causing contraction in the composite9,10 . this contraction creates mechanical stresses in the resin composite , which can disrupt the marginal seal between the composite restoration and dentin or enamel9 . polymerization shrinkage leads to several clinical problems , such as marginal discoloration , restoration fractures , solubility of the bonding system and marginal leakage12 . there are three phases in polymerization shrinkage : the pre - gel , gel point and post gel phase , but the only one capable of disrupting the marginal seal between the composite restoration and dentin / enamel is the post - gel phase10,12 . this occurs because , in the phase before the gel point , the monomers can still move or slip into new positions within the resin matrix , without causing stress at the interface12 . as the number of monomers converted into polymers increases , the flow gradually decreases , while the resin composite becomes stiffer and the material becomes strong enough to exert forces or stress in the bond system9,10 . post gel polymerization results in clinically significant stress in the composite - tooth bond and surrounding tooth structure24 , leading to adhesion failure12 . adhesive bond strength , restorative material elastic modulus , cavity design , light intensity and curing time are some of the factors influencing the marginal quality of restorations10,23 . a number of polymerization techniques have been developed in an attempt to reduce the stress caused by polymerization shrinkage . initial polymerization with low intensity light followed by final cure with high intensity light may result in improved marginal integrity without losing the achievable material properties19 . the aim of the technique using low light curing intensity is to prolong the time span before gel point is reached and to increase the material flow capability . afterwards , high light intensity is necessary to achieve complete polymerization and optimal mechanical properties10 . another technique that prolongs the time span before reaching gel point , similar to the soft start method , is the pulse activation technique16 . it differs from the soft start , as there is a waiting interval between the initial low intensity and the final exposure to the high intensity light . several studies1,21 have investigated the pulse delay technique and the general consensus is that it reduces polymerization shrinkage stress without decreasing mechanical properties . however , these two techniques have long cure times , which are inconvenient for the patient , impractical with children , uncomfortable for the dentist and make the treatment more expensive because of the extra time spent in the chair20 . a different method of providing extremely high irradiance levels is to use curing units that emit high intensity light , as they allow resin composite polymerization to occur very quickly and are recommended because of the curing depth and mechanical properties achieved . high light curing , which is compensated by the intensity , can polymerize the resin composite in a shorter time . however , these high light intensities do not allow enough flow for reducing internal stress , thus contribute to high polymerization shrinkage3,19 , and increased leakage3 . different polymerization modes can lead to the resulting polymer having different structures , even though the degree of monomer conversion is the same6 . thus , some physical properties , like hardness , diametral tensile strength and wear resistance show differences15,18 . the differences among the physical properties of composite resin cured with different polymerization systems has been shown to be more evident when this material was immersed in ethanol6,18 . ethanol causes the resin composite surface to soften , by removing the polymer structure , such as unreacted monomer , oligomers and linear polymers6 or imparting an opener structure to the polymer , facilitating the sorption of pigment agents and increasing wear5,18,22 . asmussen and hansen5 ( 1986 ) studied the relation between composite resin surface softening and discoloration , and concluded that the softening effect on a composite resin surface can increase its susceptibility to staining . material color stability over the time is an important factor in the success of an esthetic restoration19 . thus , this study investigated the effect of different polymerization modes and ethanol immersion on the composite resin susceptibility to staining . the null hypotheses to be tested were : ( 1 ) there is no difference in the susceptibility to staining of composite resin light - cured by different polymerization modes ; ( 2 ) specimens immersed in water or ethanol present no difference in the susceptibility to staining . forty cylindrical specimens were prepared using teflon ring moulds ( 5.0 mm in internal diameter and 2.0 mm depth ) held between two glass slabs separated by milar matrix strips , and then pressed with a 500 g load . the cavity was filled in one increment and was randomly polymerized according to the four experimental light curing mode groups ( table 1 ) . the initial intensity of soft start group was obtained by polymerization with the light tip at a distance of 0.9 cm from the composite resin top surface ( controlled by an electronic digital caliper ) . specimens were stored in distilled water at 37c for 24 hours and the top surface of each specimen was polished with flexible aluminum oxide disks ( sof - lex pop - on - 3 m espe ) under water spray . all specimens were randomly assigned to one of the two media ( distilled water or absolute ethanol ) at 37c for 24 hours . after this period , the specimens were washed in tap water , immersed separately in 4 ml of 2% methylene blue solution for 12 hours at 37c and thereafter , rinsed in tap water and air - dried . each specimen was weighed and ground into powder in a hard tissue mill ( marconi equip . the resulting powder was weighed , and , if the difference between the initial and final weight was greater than 10% , the specimen should be discarded . in this study the powder from each specimen was immersed for 24 hours in a glass tube containing 4 ml of absolute alcohol pa in order to dilute the methylene blue . the supernatant ( floating solution ) was analyzed with a spectrophotometer ( beckman du-65 - instruments , inc . , fullerton , ca , usa ) adjusted to a measurement wavelength of 668 nm . to calibrate the spectrophotometer , the absorbance of the standard solutions ( 0.1 ; 0.2 ; 0.3 ; 0.5 ; 1 ; 2 ; 4 ; 6 g / ml ) was determined at wavelengths ranging from 400 to 700 nm , and the maximum value was obtained at 668 nm . at this wavelength , a coefficient of linear correlation ( r=0.9998 ) and a straight - line equation ( y = a + bx ) were determined . the following relation was obtained : absorbance = 0.2716 x ( dye concentration ) - 0.0075 . to calculate the quantity of the dye concentration ( mg / ml ) in the experimental samples , the " y " was changed fo the absorbency value of each specimen . for statistical analyses , two - way anova and tukey 's test were performed on the data at 0.05 confidence level . the shapiro - wilk 's test ( test for normality ) ( p>0.05 ) confirmed the normality and homoscedasticity of the groups . anova ( table 3 ) showed no statistically significant differences ( p>0.05 ) between the two immersion media . the tukey 's test was applied to individual comparisons and showed that the soft start polymerization mode had the lowest staining means , significantly different from the high intensity and pulse - delay curing modes ( p<0.05 ) . the conventional polymerization mode did not present statistically significant differences from any other polymerization mode ( p>0.05 ) ( table 2 ) . degree of freedom ; sq  sum of squares ; mq  mean square ; - statistically significant difference ; ns - no statistically significant difference . an important factor in the success of an esthetic restoration is the material color stability over time8 . the susceptibility of a resin composite to staining may , subject to several factors , be associated with the polymerization mode . under the experimental conditions of this study , the results showed that different polymerization methods can alter composite resin susceptibility to staining . resin discoloration may be associated with its affinity for stains and water sorption due to its lower monomer conversion degree7 . water sorption is associated with polymer degradation , and can also be associated with the type of polymer11 . asmussen and petutzfeldt6 ( 2001 ) reported that light intensity and polymerization time can modify polymer structure formation . polymerization with pulse delay causes little growth at the centre of the polymer and , consequently , propagation will predominantly add one molecule of monomer after another to a growing polymer chain . this results in a more linear oligomer or polymer structure with relatively few cross - links . the final cure will activate a large part of the remaining camphoroquinone and thus give rise to a multitude of growth centers that will increase the tendency to form a branched polymer . the authors of the above - mentioned study credit the polymer structures formed with the pulse delay technique to the softness of the surface after being immersed in ethanol . this polymer structure may be associated with a greater sorption of liquid , resulting in more susceptibility to staining . the high intensity polymerization mode did not show significant differences from pulse delay and conventional polymerization . initial high intensity develops higher polymerization shrinkage stress due the rapid and immediate polymerization reaction . a high initial intensity can result in a polymer structure with higher cross - link density , due the large amount of monomer that reacts , generating several growth centers . however , a supposition for the high susceptibility to staining of this polymerization mode is that 10 seconds of polymerization are not sufficient for an effective cure at a deeper layer of the sample . aguiar , et al.1 ( 2005 ) showed that reduced polymerization times at high intensity provide an unsatisfactory cure at the deepest layer of the composite resin , and this under - cured layer is more susceptible to staining . the soft start technique showed less susceptibility to staining and it did not show statistically significant differences from conventional technique . asmussen and petutzfeldt6 ( 2001 ) reported that conventional polymerization with a continuous intensity will initiate a multitude of growth centers , resulting in a polymer structure with higher crosslink density . it may be compared with a ladder in which each original monomer molecule represents a step . in the above - mentioned study , the intensity for the conventional technique was 450 mw / cm . in the soft start groups of the present study , the initial intensity was 300 mw / cm , which is a value close to the one used in asmussen 's study6 . based on these values , it can be supposed that both soft start groups and conventional groups ( 550 mw / cm ) had comparable polymer structures and showed an equivalent behavior regarding to the susceptibility to staining . however , the conventional technique presented an intermediate behavior , and showed no statistically significant differences from either pulse delay and high intensity , or the soft start group . ethanol simulates certain beverages and alcoholic drinks , and distilled water simulates the wet intra - oral environment provided by saliva and water . it was expected that the ethanol medium could increase the resin composite susceptibility to staining . ethanol causes resin composite surface to soften , by removing the polymer structure such as unreacted monomer , oligomers and linear polymers6 , or imparting a opener structure to the polymer , thereby facilitating the sorption of pigment agents and increasing wear5,18,22 . in a composite resin with the same degree of monomer conversion , it may be that dissolution by ethanol is more selective in a relatively linear polymer than in one that is expected to have a more cross - linked structure . in other studies that associated softening agents with staining , the pigment agent was associated with the softening agent5,18,22 . aguiar , et al.2 ( 2004 ) used alcoholic and aqueous staining solutions to test susceptibility of colored restorative materials to staining , and concluded that alcoholic solutions had higher staining means than aqueous ones . in the mentioned studies , the pigment penetrated at the same time as the ethanol altered the polymeric matrix . in the present study , ethanol was first applied to the resin composite and then the pigment agent was applied . it is possible that ethanol removed the organic matrix responsible for absorbing the pigment , thus not interfering in the staining . 
 in vivo studies it is difficult to establish a direct relation between in vitro and in vivo outcomes . however , in vitro studies may guide one to conclusions that can be later confirmed in vivo and then improves the clinical activity . further studies must also been done with another dyes associated with dietary habits , such as consumption of coffee , soft drinks , alcoholic drinks and others . within the limits of this study , it may be concluded that : the ability of a composite resin surface to be stained was not related to the two immersion media studied ; the mode of polymerization can affect the staining of composite resin restorations ; the soft start polymerization mode resulted in the lowest staining for the composite resin evaluated , statistically different from high intensity and pulse delay , and similar to conventional light curing mode .