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one of the major shortcomings of all resin - based composites ( rbcs ) is that they undergo polymerization shrinkage upon curing .
the resulting volumetric contraction produces stress between the bonded restorations and tooth walls , resulting in gap formation and microleakage .
this can lead to post - operative sensitivity , recurrent caries , marginal staining , and premature failure of the restoration . over the years
, various strategies have been proposed to minimize the negative effects associated with polymerization shrinkage of rbcs .
they possess two key advantages : polymerization shrinkage lower than 1% due to ring opening during polymerization , and increased hydrophobicity due to the presence of siloxane species .
silorane - based composites have shown to exhibit good mechanical properties comparable to those of clinically successful methacrylate - based composite materials .
they have shown to possess less microleakage compared to clinically successful methacrylate - based composites .
commercially silorane - based rbc is available as filtek p90 ( 3 m espe ) .
it is a microhybrid composite , filled with fine quartz particles , and radio - opaque yttrium fluoride .
filtek p90 is used along with p90 system adhesive , a two step self - etching bonding system .
it consists of a p90 system adhesive self - etch primer which is hydrophilic and bonds to the tooth , and a p90 system adhesive bond which is hydrophobic and adheres to the resin .
although the most reliable conclusions on the performance of bonding systems in the mouth must be derived from long - term clinical trials , laboratory assays are still useful to compare their bonding performance .
bond strength testing is used as a screening tool to help understand and predict the behavior of adhesive systems .
there are conflicting reports regarding bond strengths of rbcs bonded to enamel and dentin with self - etch and total - etch adhesives .
most investigations report higher bond strength with total - etch than self - etch adhesives .
however , recent studies have concluded that self - etch adhesives may produce similar bond strengths as total - etch adhesives.[710 ] it has also been shown that different light curing unit sources can significantly influence the bond strength of composites to dentin tissue . currently , four main types of polymerization units are available : qth units , plasma arc lamps , argon laser units and light emitting diodes .
qth light source consists of a light filtered by a 100 nm bandwidth filter which transmits radiation in the 370 - 550 nm wavelength range .
leds produce a narrow band of wavelength ( 450 - 490 nm ) that is situated in the absorption spectrum of camphorquinone , and no filters are required .
the led curing lights have been found to be comparable with the halogen curing light in bond strength studies to dentin .
bond strength has been found to be more dependent on the light source used for dentin bonding agent rather than for curing resin composite .
other authors have suggested that the restorative material itself might be a more critical factor in adhesion than the curing method .
thus , this study was conducted to evaluate shear bond strength of silorane and bis - gma based composite resins using total - etch and self - etch adhesive systems , and also to compare the effect of qth and led on the shear bond strength of the experimental materials .
eighty freshly extracted human molars ( forty eight mandibular and thirty two maxillary ) were collected and stored in distilled water until use .
routine prophylactic procedure was carried out with rubber cup and pumice slurry for all teeth .
teeth samples were mounted horizontally on acrylic resin block by using a rectangular mould made of aluminium ( 522 cm ) into which self cure acrylic was mixed , followed by the teeth samples at right angle to its long axis .
occlusal enamel of teeth was removed perpendicular to the long axis of each tooth to expose a flat dentin surface at a depth of 1.5 mm , using slow - speed diamond disc with water coolant . for the composite buildup ,
a teflon mould measuring 2 mm in diameter and 3 mm in height was used to build the composite resin cylinder on the dentinal surface in a two - layer increment technique .
they were divided into four groups as follows : materials used in the study group 1 : p90 system adhesive+ filtek p90- cured with qth unit group 2 : p90 system adhesive+ filtek p90- cured with led unit group 3 : total etch adhesive+ z100- cured with qth unit group 4 : total etch adhesive+ z100- cured with led unit for groups 1 and 2 , filtek p90 self - etch primer was applied and left undisturbed for 15 s. it was air blown for 5 s and light cured for 10 s. filtek p90 bond was applied , air blown , and light cured using qth and led unit for 10 s each .
filtek p90 was placed in increments in the teflon mould and cured for 40 and 20 s respectively . for groups 3 and 4 ,
tooth surface was etched first with 35% phosphoric acid for 15 s , rinsed for 10 s and gently dried with cotton pellet .
this was followed by application of two consecutive coats of adper singlebond 2 total etch adhesive , dried with a gentle stream of air for 5 s , and light - cured using qth and led unit for 10 s each .
filtek z100 was placed in increments in the teflon mould and cured for 40 and 20 s respectively .
specimens were then subjected to shear bond strength testing using instrom universal testing machine at a crosshead speed of 1 mm per minute using a blade parallel to the adhesive interface between the adhesive and dentin .
values obtained were calculated in mega pascal ( mpa ) peak load at failure divided by the specimen surface area .
the data was subjected to one - way analysis of variance ( anova ) using spss software ( version 11.0 ) .
the multiple range test by tukey 's hsd procedure was employed to identify the significance at 5% level if the p - value by one - way anova was significant .
table 2 summarizes the mean shear bond strengths of all the groups . in the present study , p<0.05
mean shear bond strength values of the various groups the statistical analysis revealed that there is no significant difference among the shear bond strengths between groups 3 and 4 ( p=1.000 ) .
hence , type of light curing unit is not a significant factor affecting shear bond strength for z100 resin - based composite using the total - etch technique .
in contrast , the shear bond strength of group 3 is significantly higher than that of groups 1 and 2 ( p=0.000 for both ) .
similarly , the bond strength of group 4 is significantly higher than both groups 1 and 2 ( p=0.000 for both ) , thus summarizing that total - etch system gives higher shear bond strength values than self - etch system .
the bond strengths for p90 resin - based composites when cured with led curing unit ( group 2 ) was significantly lower than when cured with qth curing unit ( group 1 ) ( p=0.004 ) .
the main objective of the bond strength tests is to determine bonding of an adhesive system to the dental hard tissues .
one of the commonly used methods to test this adhesion is the shear bond strength testing . in vitro shear bond strengths are useful and essential for predicting the performance of new adhesive systems in comparison to conventional bonding systems .
total - etch and self - etch bonding systems along with two different curing units were compared for shear bond strength values in the present study . in the present study ,
the total - etch adhesive system gave a significantly higher value of shear bond strength compared to the self - etch system , which is in accordance with previous literature .
erickson et al compared three self - etch adhesive systems and eleven etch - and - rinse systems by measuring shear bond strength , and concluded that although chemical bonding may be present for some self - etch systems , it does not compete with the bond produced by total - etch systems .
self - etching priming systems combine etching and priming steps , where the primer is air dried .
this results in calcium and phosphate ions being solubilized from apatite crystals , which are suspended in alcohol and water solvents in the primer .
when these volatile solvents are evaporated , the concentration of calcium and phosphate may exceed the solubility product constants for calcium phosphate , resulting in its precipitation within the primer .
this limits ability of adhesives to penetrate the primed surface , leading to lower bond strength values . on the other hand , there are studies which show that few self - etching systems can give bond strengths comparable to the total - etch technique . these variations may be attributed to the type of bonding agent used .
other factors which may influence in vitro bond strength to dentin are the type and age of the teeth , the degree of dentin demineralization , the dentin surface being bonded , the type of bond strength test ( shear or tensile ) , the storage media , and the environmental relative humidity , which are not tested in the present study .
lower bond strength of p90 with its adhesive system may in part be attributed to differences in the chemistry of composite resins . according to guiraldo et al ,
even though there was no difference in light transmission between silorane - and methacrylate - based rbcs , there was a greater degree of subsurface polymerization of the methacrylate - based rbc . in another study ,
depth of cure and degree of conversion were found to be lower for silorane - based rbcs compared to methacrylate - based rbcs .
loading well polymerized composite resin on poorly polymerized layers can lead the restoration to bend inwards , causing marginal fracture , open margins , and cuspal deflection .
guiraldo rd et al showed that filtek p90 did not present an efficient polymerization in deeper layers , and suggested increasing exposure time or using light curing units with greater irradiance .
the lower degree of sub - surface polymerization of filtek p90 compared to the methacrylate - based composite may be a factor in the lower values of shear bond strengths shown by them .
filtek p90 has a lower filler content of 79% by weight compared to filtek z100 ( 84% by weight ) .
this may in part be responsible for higher bond strength values for methacrylate - based composite resin .
filtek p90 , a packable resin composite , is more rigid than filtek z100 , a hybrid resin composite .
thus , shrinkage stress produced during curing of packable composite resin ( filtek p90 ) may be greater , reducing adhesion to tooth structure .
this factor may have a minor role in the lower bond strengths of filtek p90 , for either light curing method .
amaral et al in a similar study concluded that hybrid resin composites had greater adhesion to dentin than packable resin composites .
it has been shown that different light sources influence restoration bond strength . in the present study , for the methacrylate rbc when used with total - etch adhesive system , there was no significant difference in the shear bond strengths between qth and led curing units .
amaral cm et al also found no significant difference in bond strengths using qth and led units .
korkmaz y and attar n also reported no significant difference in shear bond strength using self - etch adhesives , when cured with either qth or led units .
the generation of radical species for methacrylate curing is produced by camphoroquinone , which decomposes immediately due to exposure of light with a wavelength between 410 and 500 nm to start the polymerization process .
qth and led units used in this study , with a similar intensity , readily activated the photoinitiator with no significant differences in bond strengths . in a contrasting study ,
carvalho et al showed that for total - etch adhesives , polymerization with halogen light gave a higher bond strength value than those obtained with polymerization with led .
dalpino et al also reported similar results and attributed it to the lower degree of polymerization with the led unit used in their study .
in the present study , curing with led units gave a significantly lower shear bond strength compared to qth units for the silorane rbc with its adhesive system .
for qth curing unit , many of the emitted photons are out of the absorption spectrum of camphorquinone .
, the narrow wavelength is within the absorption spectrum of camphorquinone and the specific energy density for camphorquinone is higher , compensating the lower light emitted by these devices .
one component of filtek p90 initiator is camphorquinone , which matches the light spectrum of conventional light sources . in the reaction path ,
the electron donor acts in a redox process and decomposes iodonium salts into an acidic cation , which starts the ring opening polymerization process .
it is hypothesized that the narrow spectrum of led curing units may not completely activate these multiple initiators in the silorane rbc .
qth curing units with a wider bandwidth may be able to simultaneously activate all components of the p90 initiator .
this may account for the lower degree of polymerization with led unit when compared to the halogen light , thus affecting bond strength .
another reason could be that qth units have a higher heat generation , which may speed up the polymer chain induction process allowing more monomers to react before curing ends .
within the limits of this study , it can be concluded that total - etch system gave higher shear bond strength values than self - etch system .
type of light curing unit is not a significant factor affecting shear bond strength for methacrylate - based rbcs using total - etch technique .
based on the present results , it can be recommended to use conventional halogen curing units for curing the newly introduced silorane based resin composites .
however , this in vitro study needs further in vivo implementation , since factors like pulpal pressure , dentinal fluid may affect dentin bonding .
long - term clinical studies are required to evaluate the efficacy and durability of these bonding agents .
aim : to evaluate shear bond strength of silorane and bis - gma based composite resins using self - etch and total - etch adhesive systems , and compare the effect of quartz - tungten - halogen ( qth ) and light emitting diode ( led ) on the shear bond strength of the experimental materials.materials and methods : flat dentin surfaces were exposed on intact extracted molars and composite resin was built 2 mm in diameter . 
 teeth were divided randomly into four groups . 
 groups 1 and 2 were restored with p90 system adhesive and filtek p90 and cured with qth and led units respectively . 
 groups 3 and 4 were restored with total etch adhesive and filtek z100 and cured with qth and led units respectively . 
 specimens were subjected to shear bond strength testing using instrom universal testing machine.results:data was subjected to one - way analysis of variance . 
 total - etch groups gave significantly higher shear bond strength values than the self - etch groups . 
 no significant difference in shear bond strength was found between groups 3 and 4 , while group 1 showed significantly higher values than group 2.conclusion:type of light curing unit is not a significant factor affecting shear bond strength for bis - gma rbcs using total - etch technique ; while for curing silorane resin based composite ( rbcs ) , conventional halogen curing units showed better results .