a sandwich or laminate technique is one of the methods proposed for composite resin restorations , which was introduced for the first time by mclean , et al . in 1985 . the basic idea behind this technique is to use two different restorative materials for a single restorative procedure so that the maximum physico - mechanical and esthetic properties of these two materials can be exploited simultaneously .
generally , the first component is a layer of conventional or resin - modified glass - ionomer cement which has drawn attention due to its capacity to form an inherent bond with tooth structures and the resultant better seal and decrease in microleakage ( particularly in dentinal walls ) ; it can also release fluoride and decrease the odds of carious lesions .
the second component is a layer of composite materials ( including composite resin , compomer , and ormocer ) , which is used to compensate for the limitations and disadvantages of glass - ionomer cements , including weak mechanical properties and inappropriate esthetic appearance .
it has been reported that the combination of these two materials have different property results in the clinical success of restorations .
the success of the laminate technique depends on the strength of the bond between the glass - ionomer and the resin composite materials in addition to the strength of the bond between the glass - ionomer cement and dentin . however , there is a relatively weak bond strength between conventional glass - ionomers and composite resin materials , mainly because of the lack of a chemical bond between these two materials and the low cohesive strength of glass - ionomer.[35 ] several studies have evaluated different surface preparation techniques for glass - ionomers , such as the use of different bonding systems and surface etching procedures in order to increase the bond strength between these two materials in the laminate technique .
etching the surface with phosphoric acid has yielded different results . in a study carried out by sheth , et al . etching the surface of glass - ionomer had no effect on bond strength increase ; however , an increase in bond strength after acid etching has been reported in another study .
it has also been reported that premature etching of glass - ionomer cement ( before its initial setting reaction ) and failure to use an adhesive resin between the glass - ionomer cement and composite resin increases the odds of restoration failure . in another study , it was demonstrated that the use of a self - etch adhesive system on half - set glass - ionomer cement ( before its initial setting ) increases the bond strength between the glass - ionomer cement and composite resin more than that observed with the use of total - etch systems ( its application after the initial setting of the cement ) .
it has also been reported that the use of a glass - ionomer - based adhesive system applied after the cement 's initial setting improves the bond strength between the glass - ionomer cement and composite resin compared to that with the use of total - etch adhesive systems under similar conditions .
a new group of composite resin materials , giomers , have been introduced in less than a decade , which consist of reacted glass - ionomer fillers in a resin matrix ; they are used in cavities in a manner similar to composite resins with the application of an adhesive system . in addition to appropriate esthetic results , easy polishing , fluoride recharging potential and strength , these materials release fluoride which may enhance their antibacterial effects .
however , no studies to date have evaluated the bond strength between glass - ionomers and giomers ; therefore , the aim of the present study was to evaluate the effect of three surface preparation methods on the shear bond strength of giomer to different surface treated conventional glass - ionomer .
sixty cylindrical specimens were used in the present in vitro study . in order to prepare the samples ,
a plastic mold ( with an inner diameter of 6 mm and a height of 4 mm ) was placed on a glass slab .
then conventional glass - ionomer ( gc fuji ii , gc corporation , tokyo , japan ) was packed into the plastic mold after mixing according to the manufacturer 's instructions .
another glass slide was used on the other side of the mold to make the free surface of the conventional glass - ionomer smooth .
then the samples were randomly divided into three groups of 20 . according to the manufacturer
, the initial setting of gc fuji ii conventional glass - ionomer lasts in 5 minutes and a half , and the net setting time is 2 minutes and a half .
three groups were compared ; a total - etch adhesive resin or a self - etch giomer - based adhesive resin on set glass - ionomer ; or the self - etch giomer - based adhesive resin on the unset glass - ionomer ( before completion of the initial setting ) . in group 1 the surfaces of the specimens were etched with 37% phosphoric acid ( 3 m espe dental products , st .
paul , mn , usa ) for 15 seconds after 5 minutes and 30 seconds from the mixing procedure ; the initial setting reaction was confirmed with the use of a sharp dental explorer .
it should be pointed out that a complete setting of conventional glass - ionomer takes 24 - 72 hours .
after surface etching , a single bond total - etch adhesive system ( 3 m espe dental products , st .
paul , mn , usa ) was applied according to the manufacturer 's instructions and light cured [ table 1 ] .
a second transparent mold , with a diameter of 3 mm and a height of 2 mm , was placed on the conventional glass - ionomer specimen and a giomer restorative , beautifil ii a3-shade ( shofu dental corporation , osaka , japan ) , was packed into the transparent mold and cured using a halogen light - curing unit ( astralis 7 ; ivoclar vivadent , fl-9494 schaan , liechtenstein ) .
the light - directing probe had a diameter of 8 mm and directed a light ray at an intensity of 700 mw / cm perpendicular to the surface , barely touching the surface , for 40 seconds . a light intensity of 700 mw / cm was confirmed using a radiometer before the start of each experimental session .
after transparent mold removal , the specimens were cured for another 20 seconds from each direction .
then the samples were kept in humidity chamber at 37c for 1 hour before being immersed in distilled water at 37c for the next 23 hours . in the next stage ,
a 500-cycle thermocycling procedure was carried out at 5c/55c  5c with a dwell time of 30 seconds and a transfer time of 10 seconds .
chemical composition and application mode of adhesives used in group 2 , the procedures were similar to those in group 1 except that fl - bond ii ( shofu dental corporation , osaka , japan ) self - etch adhesive resin was used on set glass - ionomer surface according to the manufacturer 's instructions and light cured [ table 1 ] , without acid etching . in group 3 ,
all the procedures were similar to those in group 2 except that fl - bond ii adhesive was applied according to the manufacturer 's instructions right after the initial setting and before hardening of glass - ionomer ( 2 minutes and a half after the initiation of mixing , which is equal to the cement 's net setting time ) .
it should be pointed out that during that time the surface hardness of the cement was sufficient to place the second mold without damaging the cement . in order to measure the shear bond strength ,
the samples were mounted in cold - cured acrylic resin from the glass - ionomer side and the shear bond strength values of the samples were measured in newton using a universal testing machine ( h5k - s model , hounsfield test equipment , surrey , uk ) at a crosshead speed of 1 mm / min using a 0.5 mm - wide chisel .
then the shear bond strength values were calculated in mpa by dividing the force ( in newton ) by the surface area ( mm ) of the samples .
data were analyzed by one - way analysis of variance ( anova ) and pairwise comparisons were made by a tukey test using spss/ win.15 .
the failure modes were determined under a stereomicroscope ( smz1500 , nikon , tokyo , japan ) at 20 .
the failure modes were classified as follows : adhesive failure : failure at giomer  glass  ionomer cement interface .
in addition , to evaluate the interface between the conventional glass - ionomer and giomer in the groups under study , two additional specimens were prepared in each group and subsequent to longitudinal sectioning , they were evaluated under an scanning electron microscope [ sem ] ( tescan , vega ii xmu , brno , czech republic ) [ figure 1 ] .
scanning electron micrographs of conventional glass - ionomer and giomer interface in the study groups ( mag 500 ) .
arrows indicate margins of adhesive resin between glass - ionomer and giomer . in the three scanning electron micrographs ,
the materials which are placed on the right and left sides of the adhesive resin indicate glass - ionomer and giomer , respectively
shear bond strengths in mpa ( means and standard deviations ) for the study groups are represented in table 2 .
there were statistically significant differences in shear bond strength values between the study groups ( p < 0.0005 ) .
pairwise comparisons by a tukey test revealed that there were statistically significant differences in shear bond strengths between group 2 and the two other groups ( p < 0.0005 ) , whereas the bond strength difference between groups 1 and 3 was not statistically significant ( p = 0.609 ) . means and standard deviations ( sd ) of the shear bond strengths ( mpa ) measured in study groups
moreover , all the cohesive failures were within glass - ionomer cement and there were no cohesive failures inside the giomer .
failure modes of the study groups sem photomicrographs of the glass - ionomer and giomer interface in the study groups are represented in figure 1 . in the sem photomicrograph in group 1
[ figure 1a ] the hybrid zone ( the area of adhesive resin penetration into glass - ionomer ) is almost homogeneous but has irregular borders on the glass - ionomer cement side . in the sem photomicrograph in group 2
[ figure 1b ] the hybrid zone is regular and homogenous and in the sem photomicrograph in group 3 [ figure 1c ] the hybrid zone is irregular and non - homogenous .
moreover , the use of self - etch adhesive resin ( fl - bond ii ) in groups 2 and 3 resulted in the thicker hybrid zone than total - etch adhesive resin ( single bond ) in group 1 .
an appropriate bond between glass - ionomer and the superficial resin materials is very important for the success of the sandwich technique . in the present study shear bond strength of conventional glass - ionomer to giomer
was evaluated using three different surface preparations for glass - ionomer ( use of self - etch adhesive on glass - ionomer with or without complete initial setting reaction and use of total - etch adhesive on glass - ionomer after completion of the initial setting reaction ) . in designing the present study ,
the use of total - etch adhesive on glass - ionomer without the completion of the initial setting reaction was not considered , because in total - etch adhesives , rinsing after etching and contamination with moisture before completion of the initial setting reaction of glass - ionomer influences the surface integrity of the cement .
the results of the present study showed that shear bond strength of glass - ionomer to giomer depends on surface preparation . in this context , in the group in which glass - ionomer was set and self - etch adhesive was used , the highest shear bond strength was recorded compared to two other groups , which is consistent with the results of a study carried out by gopikrishna , et al . on glass - ionomer - based adhesives ; in the case of complete setting reaction the bond strength was higher than that in incomplete setting reaction .
contrary to the results of the present study , knight , et al . reported no statistically significant differences in the bond strength before and after initial setting of glass - ionomer in the co - cure technique , where conventional glass - ionomer , resin - modified glass - ionomer ( rmgi ) , and composite resin were placed in consecutive layers before light - curing procedure .
the differences in the results of that study and the present study might be attributed to different etching times in the two studies .
the etching time was 15 seconds in the present study , while in the study done by knight , et al .
it appears that longer etching time paves the way for greater destruction of glass - ionomer surface by the acid and decreases the bond strength .
in addition , a different technique was used in the above - mentioned study , i.e. , a layer of rmgi was used over conventional glass - ionomer before placement of composite resin and the curing process was carried out for rmgi and composite resin simultaneously .
it is well - known that the ph of glass ionomer cements is strongly acidic upon mixing , and it will increase with time , with the most rapid increase during the first 5 - 10 minutes of setting , regardless of the composition of the glass ionomer material . in comparison of groups 2 and 3 , it appears that the acidic ph of unset glass - ionomer prevented complete polymerization of the giomer - based self - etch adhesive and therefore decrease the bond strength .
in addition , this acidic ph might have an adverse effect on the polymerization of giomer itself . in a study carried out by mohamed - tahir , et al .
another study has shown that surface hardness of composite resin placed on polyalkenoate glass ( set for 4 minutes ) significantly decreased . moreover ,
sem observation of the interface in group 3 revealed an inhomogeneous structure and occasionally breakdown of the glass - ionomer surface , which may have affected the bond strength .
polyacrylic acid prevents polymerization of composite resin and results in composite resin softening ; apparently this effect was intensified in incompletely set glass - ionomer . in comparison of groups 1 and 2
, it appears that etching the glass - ionomer surface immediately after the initial setting in group 1 in the total - etch system resulted in the destruction of cement surface , crack formation and decrease in bond strength .
this speculation was confirmed by the sem micrographs of the interface . the irregular interface in group 1 may be an indication of damage and weakening of the glass ionomer surface rather than improved micromechanical interlocking , when compared to group 2 which was treated by a comparatively mild acid , i.e. , the self - etching primer .
several studies have demonstrated a decrease in bond strength of glass - ionomer to composite resin as a result of etching the glass - ionomer surface .
it has been reported that etching during the initial setting reaction of glass - ionomer leads to dissolution of weak calcium - polyacrylate rings , with the resultant deterioration of its physical properties . in the total - etch procedure
however , another reason for a higher bond strength in the self - etch group compared to the total - etch group is the fact that the acidic monomers in the self - etch primer can chemically bond to the calcium in glass - ionomer and increase bond strength .
in addition , a more appropriate compatibility of giomer with the giomer - based self - etch adhesive might be another reason for a higher bond strength in group 2 ; it has been suggested that the use of adhesives compatible with resin - based restorative materials can decrease deleterious chemical interferences .
moreover in the self - etch adhesive group , resin penetration occurs simultaneous with the etching process and it is probable that the discrepancy between these two processes is eliminated or minimized .
another factor that needs to be taken into account is the difference in composition and mechanical properties of the two adhesives ; fl - bond ii is a two - step self - etching adhesive with filler particles while single bond is a mixture of hydrophilic monomers and solvents that contains no fillers .
it has been suggested that the two - step self - etching adhesives that incorporate a hydrophobic resin as a separate bonding agent may have enhance mechanical properties compared to simplified adhesives . moreover , apart from the composition of resin matrix , addition of filler particles enhances the mechanical strength of the bonding layer and contributes to bond strength . in the present study
, no differences were observed in bond strength between groups 1 and 3 , and bond strength in both groups was significantly less than that in group 2 .
regarding failure mode in the present study , the majority of failures were of the cohesive type in glass - ionomer , which is consistent with previous studies .
this phenomenon might be attributed to lower mechanical properties of glass ionomer cements when compared to resin - based materials and the presence of numerous air inclusion bodies inside glass - ionomer , which act as stress concentration points and probably increase the odds of cohesive failure .
a disadvantage of sandwich technique is the absence of a chemical bond between conventional glass - ionomer and superficial resin materials ; therefore , researchers have focused on the establishment of a chemical bond between them .
considering the results of the present study , use of giomer - based self - etch systems not only decreases the time needed for the clinical application but also can result in the establishment of a chemical bond between giomer and conventional glass - ionomer .
taking into account the limitation of this in vitro study , etching the surface of set glass - ionomer with a total - etch system or placement of self - etch adhesive on the surface of glass - ionomer with incomplete initial setting compromised bonding of giomer to glass - ionomer .
 background : an appropriate bond between glass - ionomer and the superficial resin materials is very important for the success of sandwich technique . the aim of the present in vitro study was to evaluate the effect of three surface treatments of conventional glass - ionomer on its shear bond strength to giomer.materials and methods : sixty cylindrical specimens of a conventional glass - ionomer ( gc fuji ii ) were prepared and randomly divided into three groups ( n = 20 ) . 
 the specimens in groups 1 and 2 were treated with total - etch adhesive resin ( single bond ) along with acid etching , and self - etch adhesive resin ( fl - bond ii ) on the set glass - ionomer , respectively . 
 specimens in group 3 were treated with self - etch adhesive resin ( fl - bond ii ) before initial setting of the glass - ionomer was complete . 
 then a giomer restorative ( beautifil ii ) was added to the specimens . 
 subsequent to thermocycling , the specimens were subjected to shear bond strength test . 
 failure modes were evaluated under a stereomicroscope . 
 data were analyzed by one - way analysis of variance and a post hoc tukey test at a significance level of p < 0.05.results:there were statistically significant differences in bond strengths between the groups ( p < 0.0005 ) . 
 differences in bond strengths between group 2 and other groups were significant ( p < 0.0005 ) while the differences between groups 1 and 3 were not significant 
 . failures were predominantly of the cohesive type in all the groups.conclusion:based on the results of this study , the use of self - etch adhesive resin ( fl - bond ii ) on the set glass - ionomer yielded the highest bond strength in the glass - ionomer / giomer sandwich technique . 
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
CONCLUSION