improvements in the mechanical properties of composite resins and esthetic needs have brought about an increase in the application of these materials . weak bonding to dentin surface remains , however , is deemed as one of the major problems of composite resin restorations especially in gingival floor of cavities .
application of low elastic modulus materials such as flowable composites , glass ionomer ( gi ) or resin modified glass ionomer ( rmgi ) under the composite restorations , known as sandwich technique is one of suggested methods for improving the bond strength to dentin . in traditional sandwich technique introduced by mclean
the bond strength between gi and composite was weak and various methods have been suggested for improving the bond strength including application of acid or resin coating on unset or maturated gi . in spite of the fact that bond strength is fairly improved by these methods , low tensile strength of gi is still reported to cause cohesive fracture in gi before deboning occurred in the interface .
some studies suggest the application of rmgi instead of gi in sandwich technique due to better mechanical properties , more resistant to moisture and higher bond strength to composite , rmgi can bond chemically to composite through co - polymerization of un - reacted monomer ( hydroxyethyl methacrylate [ hema ] ) in air - inhibited layer of surperfacial surface of cured rmgi with adhesive systems or composite resins .
furthermore , it may provide covalent chemical bond between adhesive resin systems and residual monomer in polyacid chains within the cured rmgi . the total 
etch adhesive systems that need etching before applying have widely been used in restorative procedures .
the etching and rinsing processes may remove calcium and aluminum from gi and reduce the cohesive strength of it . however , there are contradictory results reported in the literature about the effects of acid etching on bond strength of rmgi to composite .
etching of rmgi may have no effect , adverse effect , or improving effect on bond strength .
the self - etch approach either two - or one - step adhesives are more user - friendly due to the time - saving and simplified procedure .
the application of self - etch adhesives on gi or rmgi improve the bond strength to composite in comparison with total etch adhesives . in co - curing technique , two different light - cured materials were coincidently polymerized .
knight el al . in their study suggested the application of co - curing technique for rmgi and composite can decrease the internal stress in composite restorations and also reduce the clinical steps .
furthermore , simultaneous curing of rmgi with composite increases the bond strength between gi and composite . a study by tulunoglu et al .
applied a co - curing technique for coincident polymerization of different adhesive systems with rmgi and concluded that although this method may increase in micro - leakage in self - etch adhesive , but , it has no significant effect in three - or two - step total - etch adhesives .
the authors assumption was that simultaneous curing of rmgi and adhesive systems may increase penetration of adhesive systems into rmgi before curing and so the bond strength will improve .
hence , the aim of the present study was to evaluate the shear bond strength ( sbs ) of rmgi to composite resin , using different adhesive systems with co-/pre - curing techniques .
the null hypotheses were as follows : 
 the application of different adhesive systems has no effect on sbs between rmgi and composite.the curing techniques ( co - or pre - curing ) have no effect on sbs . 
 
the application of different adhesive systems has no effect on sbs between rmgi and composite .
the curing techniques ( co - or pre - curing ) have no effect on sbs .
a total of 60 specimens of rmgi ( fuji ii lc , gc co ; japan ) were prepared according to manufacturer 's instructions , filled in 2 mm height of the transparent plastic rings ( 2 mm diameter  6 mm height ) and kept uncured .
the surface of rmgi samples were flattened by hand instrument and divided into two groups of 30 .
two different curing techniques were applied : in pre - curing technique , the prepared rmgi specimens were stored in a dark environment for 1 min and then light cured ( blue phase c8 , ivoclar vivadent ; schaan , lichtenstien ) at 800 mw / cm for 20 s. the specimens were randomly divided into three subgroups of ten according to adhesive systems ( total - etch , two - step self - etch and one - step self - etch ) were applied .
the adhesive systems used in this study are listed in table 1 . in the total - etch group ,
acid etch was applied for 15 s , rinsed for 10 s and air dried .
each adhesive system was used according to manufacturer 's instructions and light cured at 800 mw / cm for 20 s. the intensity of the irradiation was measured by radiometer before application . in co - curing groups , the procedure is similar to the former group , after 1 min of storage in a dark environment ; however , uncured rmgi samples are coincidently cured with adhesive systems applied on it .
then , two layers of 2 mm heliomolar composite ( ivoclar vivadent ; schaan , lichtenstien ) were placed in plastic ring and each layer was cured for 40 s. all procedures were performed by a single operator .
samples were stored in 100% humidity for 48 h and then sbs was measured by zwick universal testing machine ( zwick / z250 , type kap - z , zwick roell group ; ulm , germany ) at cross head speed of 1 mm / min .
two samples of each group were prepared for scanning electron microscope ( sem ) evaluation .
failure mode was determined by streo - microscope evaluation ( leo , 1450 up , zeiss ; oberkochen , germany ) and classified as adhesive , cohesive of composite , cohesive of rmgi and mixed .
normal distribution of the data was confirmed using the kolmogorov - smirnov test and data were analyzed using two - way anova , tukey 's post - hoc and independent t - tests with a pre - set significance level of 0.05 .
two - way anova showed that both variables ( curing technique and type of adhesive systems ) had a significant effect on sbs ( p = 0.04 for curing technique and p = 0.00 for adhesive systems ) .
tukey 's post - hoc test indicated that both self - etch adhesive systems provide higher bond strength than the total - etch adhesive system ( p < 0.05 ) while there was no significant difference between two self - etch adhesive systems .
independent t - test presented that the application of co - curing technique increased the sbs in the both of self - etch adhesive systems ( p = 0.001 for adhese and p = 0.04 for adhese one f ) but , decreased the sbs in total - etch adhesive system ( p = 0.015 ) .
the mean shear bond strength values ( mpa ) of experimental groups the distributions of failure mode in the various experimental groups are displayed in table 3 .
distribution of failure mode in experimental groups in the total - etch pre - cured group , severe crack and voids was observed in composite - adhesive interface while in the co - cured group of this adhesive , severe crack and voids appeared in rmgi - adhesive interface . in both self - etch adhesive systems either pre - cured or co - cured , no crack or void was observed in the two interfaces . in the co - cured two - step self - etch adhesive group , the obvious resin tags formation at resin - rmgi interface was observed .
two - way anova showed that both variables ( curing technique and type of adhesive systems ) had a significant effect on sbs ( p = 0.04 for curing technique and p = 0.00 for adhesive systems ) .
tukey 's post - hoc test indicated that both self - etch adhesive systems provide higher bond strength than the total - etch adhesive system ( p < 0.05 ) while there was no significant difference between two self - etch adhesive systems .
independent t - test presented that the application of co - curing technique increased the sbs in the both of self - etch adhesive systems ( p = 0.001 for adhese and p = 0.04 for adhese one f ) but , decreased the sbs in total - etch adhesive system ( p = 0.015 ) .
the distributions of failure mode in the various experimental groups are displayed in table 3 .
in the total - etch pre - cured group , severe crack and voids was observed in composite - adhesive interface while in the co - cured group of this adhesive , severe crack and voids appeared in rmgi - adhesive interface . in both self - etch adhesive systems either pre - cured or co - cured , no crack or void was observed in the two interfaces . in the co - cured two - step self - etch adhesive group , the obvious resin tags formation at resin - rmgi interface was observed .
the results of current study reject both of the hypotheses : 
 adhesive systems andcuring technique have no effect on sbs . 
 
curing technique have no effect on sbs . among the three different adhesive systems applied , the total - etch adhesive has the lowest value of sbs .
acid etching and rinsing are important factors that will possibly have significant effect on bond strength . a study by bracket and huget demonstrated that the application of acid etching can improve the bond strength between rmgi and composite .
in contrast , kerby and knobloch demonstrated this procedure can decrease the bond strength through a partial elimination of hema and un - reacted methacrylate groups in air - inhibited layer .
however , some studies showed the inhibition or decreasing of penetration of acid into rmgi due to the high resin content and formation of polymeric matrix .
therefore , they concluded that acid etching has no significant effect on bond strength of rmgi to composite .
the application of adhesive systems may improve the bond strength between composite and rmgi or traditional gi .
in the current study , both self - etch adhesive systems had higher sbs than the total - etch adhesive .
this finding is consistent with the results obtained by arora et al . and kasraie et al .
similarly , kandaswamy et al . concluded that mild self - etch adhesive had higher bond strength than the strong self - etch or total etch adhesive when they were applied on the unset gi cement .
demonstrated that the application of self - etch primer on unset gi improved the bond strength in comparison with application of this adhesive or total - etch adhesive on set gi .
the lower the viscosity adhesive used , the better the bond strength may be achieved , uncured hema in the surface of rmgi may improve the wetting ability of adhesive resins .
the observation of noticeable resin tags formation in sem micrographs of co - cured two - step self - etch adhesive group may be due to lower viscosity or better wet ability of this adhesive .
the penetration of resin into rmgis may improve the strength of these cements and therefore , the failure mode may change .
evaluation of the failure mode in this group showed that the number of cohesive failure of rmgi decreased and mixed failure was the most frequently failure mode was observed . in the total - etch co - cured group ,
acid was applied before formation of the resinous matrix so , acid could penetrate more into rmgi .
the formation of weak salt on the surface of rmgi , removing or decreasing calcium and aluminum ions that reduce the tensile strength of rmgi and reduction in hema content may be explanations for reduction of bond strength in this experimental group .
furthermore , the increase of the cohesive failure mode in this group , in comparison with the other groups , points out the weakening effect of acid application on the uncured rmgi structure .
sem analysis showed the cracks and voids in rmgi - resin adhesive interface that further confirmed this hypothesis .
there are contradictions about the failure mode observed in the previous studies since they reported the cohesive failure of gi / rmgi , mixed or adhesive as the most failure mode observed . in the current study , the most of specimens in total - etch co - cured group fractured cohesively in rmgi that remarks the detrimental effect of acid etching on uncured rmgi . in other groups , adhesive or mixed failure modes
were the most observed failure modes . in sem micrographs of two self - etch groups , no cracks and voids in any interfaces
gopikrishna et al . concluded that the carboxylic groups in self - etch primer can chemically bond with calcium in gi as they bond to calcium of hydroxyapatite in dentin . the other factor to keep in mind is that leaching the metallic ions during the etching and rinsing procedure can decrease the mechanical properties of gi .
hence , in the current study acid - etch was applied after 1 min - that 's an acceptable time for clinical use - until acid - base reaction induced rather initial setting and reduced the leaching the metallic ions .
thickness and tensile strength of gi cement , type , viscosity and stress polymerization of composite , handling procedure and total removing the residual acid ( if acid etch was applied ) are other certain factors may affect the bond strength between gi or rmgi and the composite . in the current study ,
all adhesives are chosen from the same manufacturer , so that they will be only slightly different in chemical compositions that may affect the bond strength . in this condition
, different application methods of these adhesives may have more effect on sbs than the little chemical composition differences .
the application of co - curing technique and self - etch adhesive systems may improve the sbs between composite and rmgi .
the authors suggested to evaluate the effect of the co - curing technique and the self - etch adhesive in clinical situation in future research .
furthermore , the assessment of the application of co - curing technique on bond strength of gi based - adhesives is required .
 aim : to evaluate bond strength between rmgi and composite using different adhesive systems and curing techniques.materials and methods : sixty prepared samples of rmgi were randomly divided into six groups according to adhesive systems ( total - etch , two - step self - etch and all - in - one ) and curing techniques ( co - curing and pre - curing ) . in co - curing technique , the adhesive systems were applied on uncured rmgi samples and co - cured together . in the pre - curing technique , before application of adhesive systems , the rmgi samples were cured . 
 composite layers were applied and shear bond strength was measured . 
 two samples of each group were evaluated by sem . 
 failure mode was determined by streomicroscope.results:both curing methods and adhesive systems had significant effect on bond strength ( p - value < 0.05 ) . 
 there was an interaction between two factors ( p - value < 0.05 ) . 
 both self - etch adhesives had significantly higher shear bond strength than the total - etch adhesive ( p - value < 0.05 ) . 
 the co - curing technique improved the bond strength in self - etch adhesives , but decreased the bond strength in total - etch adhesive ( p - value<0.05).conclusion : the application of self - etch adhesive systems and co - curing technique can improve the bond strength between the rmgi and composite . 
INTRODUCTION
MATERIALS AND METHODS
RESULTS
Bond strength
Failure modes
SEM analysis
DISCUSSION
CONCLUSION