composite resins are made up of a complex mixture of the organic resin matrix , inorganic fillers , and a silane coupling agent connecting the two . base monomers such as bisphenol a - glycidyl methacrylate ( bisgma ) and urethane dimethacrylate ( udma ) along with low - viscosity monomer like triethylene glycol dimethacrylate ( tegdma ) constitute the polymerizable resin matrix . this technique is time - consuming , and there is a risk of contamination while placing , adapting , and curing each increment . its unique advantage is that it can be placed and cured in an increment of 4-mm thickness , without adverse effect on polymerization shrinkage . one of the alternatives to overcome polymerization shrinkage is the use of soft - start curing which involves , low - intensity light during the initial phase of polymerization and later switched to high - intensity light for the remaining curing time reducing the rate of polymerization . the elution of residual monomers , oligomers , and other degradation products into the oral environment has raised concerns regarding composites . it occurs by diffusion of the resin matrix or by its degradation or erosion over a period . according to ferracane , the factors affecting elution are : ( i ) amount of polymerization reaction , ( ii ) chemistry of the solvent , and ( iii ) chemical nature and size of the released components . the unpolymerized monomers can get eluted directly into the pulp by means of dentinal microchannels . bisgma has a strong hemolytic potency which can cause time- and concentration - dependent cytotoxicity in human gingival and pulp fibroblasts . tegdma can easily penetrate cell membranes and react with intracellular molecules , thereby causing cytotoxicity . thus , the aim of this study was to evaluate the amount of the elution of bisgma and tegdma from these composite resins cured by a quartz - tungsten - halogen ( qth ) light using  soft start  curing technique , when stored at a storage period of 24 h and 1 week using two polymerization times : 30 and 40 s. two nanohybrid dimethacrylate bulk - fill composite resins were used [ table 1 ] . from each of them , 34 samples were prepared . a total of 68 samples . these samples were divided into two groups of 17 samples each based on different polymerization times : 30 and 40 s. materials used for the study 
 group 1 : 30 s polymerization ( 30 s)group 2 : 40 s polymerization ( 40 s ) . group 1 : 30 s polymerization ( 30 s ) group 2 : 40 s polymerization ( 40 s ) . a standard teflon mold of 2 mm  2 mm  2 mm dimension was used to prepare the specimens . it was placed on a glass plate , and underneath the mold , a transparent plastic matrix strip was positioned . the material was inserted into the mold followed by the placement of one more transparent plastic matrix strip over them , to prevent oxidation of the superficial layer . each specimen was cured using  soft start curing technique  with qth curing units for 30 and 40 s , i.e , initial curing with the low intensity of 450 mw / cm for 10 s using qhl-75 curing light ( dentsply ) and the remaining 20 and 30 s , respectively , were cured with a higher intensity of 750 mw / cm using astralis 7 curing light ( ivoclar vivadent ) . a calibrated radiometer system was used to verify the irradiance at each use of the light cure unit . a 1-cm glass plate was used to standardize the distance between the light source and the specimen . immediately after curing , the specimens were immersed in 2 ml of 99.99% ethanol ( hayman limited , eastways park , witham , essex , cm88ye , england ) . the storage of these samples was done for a time period of 24 h at room temperature . after 24 h , the storage medium was changed , and the specimens were again stored for 7 days . after 24 h and 7 days , the samples were measured using the high - performance liquid chromatography ( hplc ) unit ( shimadzu , model spd 20a , shimadzu corporation , kyoto , japan ) . the separation of monomers took place with a cc 125/4 nucleodur 100 - 5 c18ec hplc - column . the mobile phase was acetonitrile / water ( 75/25% v / v ) at a flow rate of 1 ml / min , and detection was performed at a wavelength of 254 nm for 30 min . for the analysis of extracted residual monomers , reference standards of bisgma ( cas no . 261548 , sigma - aldrich chemical co. , usa ) and tegdma ( cas no . 494356 , sigma - aldrich chemical co. , usa ) were purchased and stock solutions were prepared by appropriate quantitative dilution . twenty microliters from the solution was injected into hplc system and standard chromatograms were obtained for both the monomers individually . the acquired data were statistically analyzed using student 's t - test and paired t - test . statistical package for social science software version 15 ( ibm spss statistics , ny , usa ) was used . two nanohybrid dimethacrylate bulk - fill composite resins were used [ table 1 ] . from each of them , 34 samples were prepared . a total of 68 samples . these samples were divided into two groups of 17 samples each based on different polymerization times : 30 and 40 s. materials used for the study 
 group 1 : 30 s polymerization ( 30 s)group 2 : 40 s polymerization ( 40 s ) . group 1 : 30 s polymerization ( 30 s ) group 2 : 40 s polymerization ( 40 s ) . a standard teflon mold of 2 mm  2 mm  2 mm dimension was used to prepare the specimens . it was placed on a glass plate , and underneath the mold , a transparent plastic matrix strip was positioned . the material was inserted into the mold followed by the placement of one more transparent plastic matrix strip over them , to prevent oxidation of the superficial layer . each specimen was cured using  soft start curing technique  with qth curing units for 30 and 40 s , i.e , initial curing with the low intensity of 450 mw / cm for 10 s using qhl-75 curing light ( dentsply ) and the remaining 20 and 30 s , respectively , were cured with a higher intensity of 750 mw / cm using astralis 7 curing light ( ivoclar vivadent ) . a calibrated radiometer system was used to verify the irradiance at each use of the light cure unit . a 1-cm glass plate was used to standardize the distance between the light source and the specimen . immediately after curing , the specimens were immersed in 2 ml of 99.99% ethanol ( hayman limited , eastways park , witham , essex , cm88ye , england ) . the storage of these samples was done for a time period of 24 h at room temperature . after 24 h , the storage medium was changed , and the specimens were again stored for 7 days . after 24 h and 7 days , the samples were measured using the high - performance liquid chromatography ( hplc ) unit ( shimadzu , model spd 20a , shimadzu corporation , kyoto , japan ) . the separation of monomers took place with a cc 125/4 nucleodur 100 - 5 c18ec hplc - column . the mobile phase was acetonitrile / water ( 75/25% v / v ) at a flow rate of 1 ml / min , and detection was performed at a wavelength of 254 nm for 30 min . for the analysis of extracted residual monomers , reference standards of bisgma ( cas no . 261548 , sigma - aldrich chemical co. , usa ) and tegdma ( cas no . 494356 , sigma - aldrich chemical co. , usa ) were purchased and stock solutions were prepared by appropriate quantitative dilution . twenty microliters from the solution was injected into hplc system and standard chromatograms were obtained for both the monomers individually . the acquired data were statistically analyzed using student 's t - test and paired t - test . statistical package for social science software version 15 significantly higher amount of bisgma was eluted from tetric n - ceram bulk - fill composite at the end of 24 h as compared to 1 week when cured for 30 s , i.e , p < furthermore , significantly higher amount of bisgma was eluted from everx posterior composite at the end of 24 h as compared to 1 week when cured for 30 s , i.e , p < 0.001 and 40 s , i.e , p = 0.022 < 0.05 . on comparing the elution of tegdma from everx posterior , significantly higher amount was eluted at the end of 24 h as compared to 1 week when cured for 30 s , i.e , p < 0.001 and 40 s , i.e , p < 0.001 [ table 2 ] . paired t - test was applied to compare the mean amount of release of bisphenol a - glycidyl methacrylate from tetric n - ceram bulk fill and everx posterior and triethylene glycol dimethacrylate from everx posterior based on different storage times student 's t - test showed that from everx posterior , a higher amount of tegdma was released at the end of 24 h as compared to bisgma , when cured for 30 s , i.e. p = 0.166 and when cured for 40 s , i.e. p = 0.456 , but it was not statistically significant . at the end of 1 week , significantly higher amount of bisgma was released as compared to tegdma , when cured for 30 s , i.e , p < 0.001 and when cured for 40 s , i.e , p < 0.001 [ table 3 ] . student 's t - test was applied to the mean amount of release of bisphenol a - glycidyl methacrylate and triethylene glycol dimethacrylate from everx posterior bulk - fill composite based on different storage times asmussen and peutzfeldt reported that a linear polymer structure with a fewer cross - links is significantly more prone to softening in ethanol . studies show the cytotoxic potency of basic monomers to be in the following order : bisgma > udma > tegdma > > > hema . according to the national institute for occupational safety and health , since tegdma is lipophilic nature , it can penetrate the cytosol and membrane lipid compartments of mammalian cells . 99.99% ethanol was used as a solvent , owing to its maximum ability to extract residual monomers and solubility parameter similar to bisgma . the maximum elution , i.e. 85100% of monomers was reported to occur within 24 h by ferracane and condon and within 7 days by ortengren et al . hence , the elution of monomers after polymerization was tested at the end of 24 h and 7 days in the present study . the molecules of bisgma and udma , which are high molecular weight base monomers , get decompose in the gas chromatograph and , hence , only their decomposition products can be evaluated . hplc , being an accurate and common separation method , was preferred to qualify and quantify the elutable monomers . in the present study , the overall elution of bisgma was higher than tegdma , for both composite materials [ table 2 ] , which correlates with the results of komurcuoglu et al . predominantly , it could be because bisgma had low double - bond conversion as compared to tegdma or due to variation in the reactive potentials and chemical properties of the two . in addition to the above , 99.99% ethanol used for the storage of the samples could also be one of the probable reasons for higher elution of bisgma . when the elution of bisgma from both the bulk - fill nanohybrid dimethacrylate resins was compared , paired t- test revealed that 24 h samples showed significantly higher release at p < 0.05 , irrespective of the different polymerization time . the release of tegdma was also significantly higher from the 24 h samples , irrespective of the curing time [ table 2 ] , and is in accordance with ferracane and condon . varying polymerization time groups of 30 and 40 s showed no significant difference in the elution of bisgma and tegdma from the two bulk - fill composite resins [ graph 1 ] . bar graph depicting the mean amount of the release of bisphenol a - glycidyl methacrylate from tetric n - ceram bulk fill and everx posterior and triethylene glycol dimethacrylate from everx posterior based on different polymerization times at the end of 24 h , a higher amount of tegdma was eluted from everx posterior as compared to bisgma but not significantly , i.e , p = 0.166 ( p > 0.05 ) when cured for 30 s and p = 0.456 ( p > 0.05 ) when cured for 40 s [ table 3 ] . nathanson et al . , who have shown that within the first 4 min , the maximum amount of tegdma was released which reduced thereafter . in addition , durner et al . reported that initially , the elution of tegdma was higher than bisgma . this could be because of its hydrophilicity and small molecular weight , exhibiting higher mobility . while at the end of 7 days , the elution of bisgma from the same material was significantly higher than tegdma , irrespective of the curing times , i.e , p < 0.05 [ table 3 ] , which is similar to the results of the study by polydorou et al . of the two bulk - fill composites evaluated , tetric n - ceram bulk fill showed higher elution of bisgma than everx posterior , irrespective of the varying polymerization and storage time . this might be due to the absence of tegdma . furthermore , it contains prepolymer fillers , due to which any remaining unreacted or pendant c = c double bond increases the amount of elutable monomers [ graph 2 ] . bar graph depicting the mean amount of the release of bisphenol a - glycidyl methacrylate between tetric n - ceram bulk fill and everx posterior composite resins within the limitations of the study , the following conclusions were drawn : 
 for bisgma and tegdma elution , a significantly higher amount of monomer was eluted after 24 h than after 1 weekhigher quantity of bisgma was eluted from tetric n - ceram bulk fill in comparison to everx posterior , irrespective of the polymerization time and storage periodsalthough samples were cured using soft - start curing technique , the elution of monomers from bulk - fill composites was still observed . 
  for bisgma and tegdma elution , a significantly higher amount of monomer was eluted after 24 h than after 1 week higher quantity of bisgma was eluted from tetric n - ceram bulk fill in comparison to everx posterior , irrespective of the polymerization time and storage periods although samples were cured using soft - start curing technique , the elution of monomers from bulk - fill composites was still observed . manufacturers recommend 20 s curing time with qth unit to polymerize bulk - fill composites . however , from the present study , even though 30 and 40 s of curing time , composite resins showed no notable effect in the elution of monomers . further considerations and research on suitable modifications in the composite materials , curing lights , or curing techniques to reduce monomer elution from composite resins are required . residual monomers were eluted from bulk - fill composite resins at all time periods , even when cured using an alternative curing technique like  soft start  curing technique . since absolute alcohol was used to immerse the cured composite samples , it did not simulate the oral cavity environment , but quantifies the amount of residual monomer which can be correlated . residual monomers were eluted from bulk - fill composite resins at all time periods , even when cured using an alternative curing technique like  soft start  curing technique . since absolute alcohol was used to immerse the cured composite samples , it did not simulate the oral cavity environment , but quantifies the amount of residual monomer which can be correlated .
aim and objectives : to evaluate the elution of bisphenol a - glycidyl methacrylate ( bisgma ) and triethylene glycol dimethacrylate ( tegdma ) from two bulk - fill composites at different polymerization times , for different storage periods when cured with quartz - tungsten - halogen ( qth ) curing unit.materials and methods : tetric n - ceram bulk fill and everx posterior were analyzed using high - performance liquid chromatography unit . 
 totally , 68 samples were prepared , two groups ( n = 17 ) from both the composites , one for each tested polymerization time . 
 each sample was cured with a qth curing unit , using soft - start curing technique and stored in 2 ml of ethanol for 24 h. storage medium was renewed and then stored again for 1 week . 
 data acquired were statistically analyzed.results:the elution of bisgma was significantly higher from tetric n - ceram bulk fill and bisgma and tegdma from everx posterior composite at the end of 24 h , irrespective of the curing time . 
 in everx posterior , a higher amount of tegdma was eluted at the end of 24 h , while at the end of 1 week , significantly higher amount of bisgma was released.conclusion:a significant amount of the release of bisgma as well as tegdma was seen from both the composites when stored for different time intervals .