since wilson and kent s introduction of glass ionomer cement ( gic ) during the early 70 s , several modifications have been made to improve its properties.1 glass ionomer cements are considered the most effective biomaterial that bond to hard tooth substrates.2,3 comparing the retention rates of class v adhesive restorations , glass ionomers showed the most effective and durable bond to tooth structure.4 resin - modified glass ionomer ( rmgi ) is a combination of glass ionomer fillers and water - soluble monomer in an aqueous solution of polyacrylic acid.5 while conventional gis bond to tooth substrate by ion - exchange,6 rmgis bond to tooth substrates through both ion - exchange and micromechanical interlock.7,8 a newly introduced nano - filled rmgi restorative ( ketac nano-100 , 3m - espe ) undergoes both glass ionomer and free radical reactions . according to its manufacturer , ketac nano-100 restorative is a new technical development that combines the benefits of a resin modified light cure glass ionomer and bonded nano - filler technology  . nano - filled rmgi is a light - cured paste / paste system that is delivered from a multi - dose its chemistry is based on metacrylate modified polyalkenoic acid , which was firstly employed in vitrebond liner / base material ( ketac n-100 , technical product profile , 3 m - espe ) . nano - primer is a one - component acidic primer , which is essential to bond the nano - filled rmgi to dentin ( ketac nano-100 technical product profile ) and it does not require the pre - conditioning step of the dentin surface.3 the manufacturer claims that the bond strength of nano - filled rmgi to dentin using the nano - primer is equivalent to the greatest number of gis used . this claim was not confirmed in the study by coutinho et al.3 conventional rmgi ( fuji ii lc ) showed a significantly higher tbs to both enamel and dentin than that recorded with nano - filled rmgi . the interaction of acids to tooth structure has been evaluated extensively ; nevertheless , it has never been elucidated.9 the use of different dentin conditioners did not only improve the bond strength of self - etching primers to dentin,10,11 but also improved the shear bond strength ( sbs ) of the glass ionomer adhesive.12 accordingly , the purpose of this study was to evaluate the effect of the pre - conditioning step on the sbs of nano - filled rmgi to dentin . the null hypothesis tested is that the pre - conditioning step will not affect the sbs of nano - filled rmgi to dentin . twenty - four caries - free non - restored human molars were used in this study . the teeth were collected and stored in a 0.1% thymol solution for not more than one month . each tooth was sectioned in a mesio - distal direction down to the root into two halves , using a low - speed diamond disc under copious water irrigation ; then , the two halves were completely separated using a surgical chisel,12 yielding 48 halves . each half was fixed from its dentin side to the base of a metallic rectangular mould with double - faced adhesive tape . auto - cured acrylic resin material ( acroston , under exclusive license of acroston dental company - cairo , egypt ) was poured to completely cover each half . after hardening of the acrylic resin , each dentin surface was wet ground using 600 grit sic papers for 60 seconds to create a standardized smear layer . the 48-halves were equally divided into 6 groups ( n=8 ) according to the dentin surface treatment procedures . materials , compositions and manufacturers are summarized in table 1 . before the application of each dentin treatment , double - faced adhesive tape , with a hole of 3 mm in diameter nano - primer was applied for 15 seconds , air - dried using gentle , compressed - air for 10 seconds , and light cured for 10 seconds with a halogen light - curing unit ( hilux , dental curing light unit , benlioglu ; ankara , turkey ) with an output of 600 mw / cm . before the application of the nano - filled rmgi , a split teflon mould , 2 mm in thickness with a central hole of 3 mm in diameter , was fixed over the dentin surface . the material was dispensed over a pad with the aid of its clicker dispenser , mixed using a metallic spatula for 20 seconds , and applied over the cured primer , then , the material was light cured for 20 seconds . group 2 : nano - filled rmgi was directly applied over the dentin surface , without the application of the nano - primer and light cured as in group 1 . group 3 : the dentin surface was conditioned using 25% polyacrylic acid for 10 seconds , rinsed with a copious air / water spray for 10 seconds and , blot dried using sterile cotton , leaving the dentin surface visibly moist ( wet bonding ) . nano - filled rmgi was applied directly over the dentin surface , without the application of the nano - primer and light cured , as in group 1 . group 4 : the dentin surface was conditioned with 25% polyacrylic acid as in group 3 . nano - primer and nano - filled rmgi were applied as described in group 1 . group 5 : the dentin surface was etched using 35% phosphoric acid for 15 seconds , rinsed for 20 seconds and blot dried as in group 3 . nano - primer and nano - filled rmgi were applied as described for group 1 . group 6 : the dentin surface was conditioned with edta solution for 60 seconds , rinsed for 20 seconds and blot dried as in group 3 . nano - primer and nano - filled rmgi were applied as described for group 1 . subsequent to light curing of the nano - filled rmgi , the teflon mould was dissembled , and the dentin surfaces- with their attached rmgi cylinders- were stored in distilled water for 24 hours at 37c . after 24 hours , each specimen was mounted to the universal testing machine ( lr5k series , lloyd instrument ; fareham , uk ) . a specially fabricated , metallic , chisel - bladed instrument was positioned as accurately as possible on the nano - filled rmgi / dentin interface . the test was run at a crosshead speed of 0.5 mm / minute until failure . the load recorded in newton was divided over the calculated surface area and the sbs was retrieved in mpa . groups 2 and 3 were not tested , as all specimens of these two groups failed before testing during the 24-hour storage period , and their sbs was expressed as 0.0 mpa for statistical analysis . an additional eight human molars were included to evaluate the nano - filled rmgi / dentin interface for groups 1 , 4 , 5 and 6 , two teeth per group . the occlusal enamel of each tooth was wet ground using the laboratory trimmer to expose a flat dentin surface surrounded by enamel . subsequent to enamel removal , the dentin surfaces were wet ground using 600 grit sic paper for 60 seconds , in order to create a standardized smear layer . the teeth of the above mentioned groups ( groups 1 , 4 , 5 and 6 ) were treated as described for sbs testing . after each treatment , an approximately 2 mm thick nano - filled rmgi block was applied and light cured for 20 seconds . after 24-hour storage period in distilled water at 37c , each tooth was sectioned mesio - distally using the low speed diamond disc under copious water irrigation to the level of the cervical line into two halves . the evaluated surfaces were wet ground with 600 , 800 , 1000 grits and finally with 1200 grit sic papers . the evaluated surfaces were acid etched with 35% phosphoric acid for 15 seconds , rinsed for 20 seconds and , then , gently air dried . the specimens were immersed in 5% sodium hypochlorite for 120 seconds , and washed under running water for 5 minutes . the specimens were dehydrated in ascending concentration of ethanol , 50% , 70% , 90% for 20 minutes each and , then immersed in 100% for 1 hour . the specimens were fixed on specimen s holder using double - faced stickers and sputter coated ( bal - tec , scd 005 sputter coater , germany ) . nano - filled rmgi / dentin interface was evaluated at 2000x using sem ( philips xl series , xl30 , holland ) operated at 12 kv . one way analysis of variance ( procedure anova of sas ) followed by duncan s multiple range test were used to test the effect of dentin conditioners on sbs . the 48-halves were equally divided into 6 groups ( n=8 ) according to the dentin surface treatment procedures . materials , compositions and manufacturers are summarized in table 1 . before the application of each dentin treatment , double - faced adhesive tape , with a hole of 3 mm in diameter nano - primer was applied for 15 seconds , air - dried using gentle , compressed - air for 10 seconds , and light cured for 10 seconds with a halogen light - curing unit ( hilux , dental curing light unit , benlioglu ; ankara , turkey ) with an output of 600 mw / cm . before the application of the nano - filled rmgi , a split teflon mould , 2 mm in thickness with a central hole of 3 mm in diameter , was fixed over the dentin surface . the material was dispensed over a pad with the aid of its clicker dispenser , mixed using a metallic spatula for 20 seconds , and applied over the cured primer , then , the material was light cured for 20 seconds . group 2 : nano - filled rmgi was directly applied over the dentin surface , without the application of the nano - primer and light cured as in group 1 . group 3 : the dentin surface was conditioned using 25% polyacrylic acid for 10 seconds , rinsed with a copious air / water spray for 10 seconds and , blot dried using sterile cotton , leaving the dentin surface visibly moist ( wet bonding ) . nano - filled rmgi was applied directly over the dentin surface , without the application of the nano - primer and light cured , as in group 1 . group 4 : the dentin surface was conditioned with 25% polyacrylic acid as in group 3 . nano - primer and nano - filled rmgi were applied as described in group 1 . group 5 : the dentin surface was etched using 35% phosphoric acid for 15 seconds , rinsed for 20 seconds and blot dried as in group 3 . nano - primer and nano - filled rmgi were applied as described for group 1 . group 6 : the dentin surface was conditioned with edta solution for 60 seconds , rinsed for 20 seconds and blot dried as in group 3 . nano - primer and nano - filled rmgi were applied as described for group 1 . subsequent to light curing of the nano - filled rmgi , the teflon mould was dissembled , and the dentin surfaces- with their attached rmgi cylinders- were stored in distilled water for 24 hours at 37c . after 24 hours , each specimen was mounted to the universal testing machine ( lr5k series , lloyd instrument ; fareham , uk ) . a specially fabricated , metallic , chisel - bladed instrument was positioned as accurately as possible on the nano - filled rmgi / dentin interface . the test was run at a crosshead speed of 0.5 mm / minute until failure . the load recorded in newton was divided over the calculated surface area and the sbs was retrieved in mpa . groups 2 and 3 were not tested , as all specimens of these two groups failed before testing during the 24-hour storage period , and their sbs was expressed as 0.0 mpa for statistical analysis . an additional eight human molars were included to evaluate the nano - filled rmgi / dentin interface for groups 1 , 4 , 5 and 6 , two teeth per group . the occlusal enamel of each tooth was wet ground using the laboratory trimmer to expose a flat dentin surface surrounded by enamel . subsequent to enamel removal , the dentin surfaces were wet ground using 600 grit sic paper for 60 seconds , in order to create a standardized smear layer . the teeth of the above mentioned groups ( groups 1 , 4 , 5 and 6 ) were treated as described for sbs testing . after each treatment , an approximately 2 mm thick nano - filled rmgi block was applied and light cured for 20 seconds . after 24-hour storage period in distilled water at 37c , each tooth was sectioned mesio - distally using the low speed diamond disc under copious water irrigation to the level of the cervical line into two halves . the evaluated surfaces were wet ground with 600 , 800 , 1000 grits and finally with 1200 grit sic papers . the evaluated surfaces were acid etched with 35% phosphoric acid for 15 seconds , rinsed for 20 seconds and , then , gently air dried . the specimens were immersed in 5% sodium hypochlorite for 120 seconds , and washed under running water for 5 minutes . the specimens were dehydrated in ascending concentration of ethanol , 50% , 70% , 90% for 20 minutes each and , then immersed in 100% for 1 hour . the specimens were fixed on specimen s holder using double - faced stickers and sputter coated ( bal - tec , scd 005 sputter coater , germany ) . nano - filled rmgi / dentin interface was evaluated at 2000x using sem ( philips xl series , xl30 , holland ) operated at 12 kv . one way analysis of variance ( procedure anova of sas ) followed by duncan s multiple range test were used to test the effect of dentin conditioners on sbs . one way anova followed by duncan s multiple range test revealed that the pre - conditioning step using 35% phosphoric acid and edta solution ( groups 5 and 6 , respectively ) yielded statistically significant higher sbs compared to all tested groups ( 12.01.8 mpa and 11.94.4 mpa , respectively ) . when the dentin was preconditioned with 25% polyacrylic acid ( group 4 , 8.63.1 mpa ) , it reported a statistically significant higher sbs than that when the nano - filled rmgi was applied according to manufacturer instructions ( group 1 , 5.52.2 mpa ) . sbs of nano - filled rmgi when directly applied either over dentin surface ( group 2 ) or over conditioned dentin with 25% polyacrylic acid ( group 3 ) showed significant differences between all groups ( 0.00.0 mpa and 0.00.0 mpa , respectively ) , which exhibited 100% pre - test failure in all their specimens . sem evaluation of nano - filled rmgi / dentin interface , when the material was applied according to manufacturer instructions ( group 1 ) revealed no evidence of hybrid layer formation or resin tag extensions . a gap was detected between the restoration and underlying dentin , and the smear layer was observed over the dentin surface ( figure 1 ) . when the dentin surface was pre - conditioned with 25% polyacrylic acid prior to the use of the nano - primer ( group 4 ) , few discrete and very shallow resin tags extensions were noticed , although a thin hybrid layer was observed ( figure 2 ) . numerous long , funnel - shaped , resin tag extensions were detected when the dentin surface was pre - conditioned with 35% phosphoric acid prior to the application of the nano - primer ( group 5 ) . a thick hybrid layer was evident with filler distribution at the bottom and within the hybrid layer , as well as around the orifices of the dentinal tubules ( figure 3 ) . long and thin resin tag extensions were noticed when the dentin surfaces were pre - conditioned with edta solution for 60 seconds prior to the application of the nano - primer ( group 6 ) . the hybrid layer was thinner than that detected with group 5 , with a few discrete filler distributions ( figure 4 ) . sem evaluation of nano - filled rmgi / dentin interface , when the material was applied according to manufacturer instructions ( group 1 ) revealed no evidence of hybrid layer formation or resin tag extensions . a gap was detected between the restoration and underlying dentin , and the smear layer was observed over the dentin surface ( figure 1 ) . when the dentin surface was pre - conditioned with 25% polyacrylic acid prior to the use of the nano - primer ( group 4 ) , few discrete and very shallow resin tags extensions were noticed , although a thin hybrid layer was observed ( figure 2 ) . numerous long , funnel - shaped , resin tag extensions were detected when the dentin surface was pre - conditioned with 35% phosphoric acid prior to the application of the nano - primer ( group 5 ) . a thick hybrid layer was evident with filler distribution at the bottom and within the hybrid layer , as well as around the orifices of the dentinal tubules ( figure 3 ) . long and thin resin tag extensions were noticed when the dentin surfaces were pre - conditioned with edta solution for 60 seconds prior to the application of the nano - primer ( group 6 ) . the hybrid layer was thinner than that detected with group 5 , with a few discrete filler distributions ( figure 4 ) . self - adhesiveness properties to tooth structure3 , anticariogenicity13 and minimal tooth / restoration interface microleakage14 are among the advantages presented by glass ionomer cements . the introduction of rmgis was aimed to overcome the drawbacks encountered with conventional gis.15 a new approach to improve some properties of conventional rmgis is the introduction of a nano - filled rmgi ( ketac nano-100 , technical material profile ) . this new material requires the application of nano - primer without the need for a pre - conditioning step . the use of both mild and aggressive conditioners in this study was aimed to evaluate whether the pre - conditioning step can improve the bond strength and facilitate the hybridization of the nano - primer to dentin . the pre - conditioning step not only significantly improved the sbs of nano - filled rmgi , but also allowed for hybridization to dentin . in this study , it was reported that when the dentinal tubules were cut obliquely , rmgi adhesive failed to form resin tags.13 in the occlusal dentin , the dentinal tubules were cut with an open end ,  which might facilitate the penetration of the resin in the acidic primer to form resin tag extensions . although , the shear test , does not actually assess the adhesive bond,16 it is thus far the most popular technique to measure bond strength . the shear test is the easiest and fastest method and does not require further specimen preparations after the completion of bonding procedures.17 sem in this study showed the presence of a smear layer over the dentin surface and the lack of hybridization formation ( figure 1 ) , when the nano - filled rmgi was applied according to manufacturer instructions . this could be attributed to the inability of nano - primer to decalcify the underlying dentin , which might be due to the high ph of the nano - primer ( 3 ) . our result confirmed the finding reported by coutinho et al,3 despite the differences in evaluation methodology between the two studies . they reported that nano - primer could not decalcify the underlying dentin surface , resulting in no evidence of hybridization . the gap noticed in semicrographs ( figure 1 ) could be attributed to the lack of hybridization and resin tag extensions in the underlying dentin , where contraction of dentin could be expected during specimen processing for sem evaluation , which in turn allows for the separation of such material at its weaker point . in this study , the significant improvement in the sbs of nano - filled rmgi with the pre - conditioning step could be explained by the sem results ( figures 2 , 3 and 4 ) . it was reported that polyacrylic acid removed the smear layer without widely opening the dentinal tubules.18 discrete and short resin tags , with the formation of the thin hybrid layer ( figure 2 ) , might cause the significant improvement in sbs when 25% polyacrylic acid was used . regardless of the difference in the aggressiveness of phosphoric acid1921 and sem showed that ( figures 3 and 4 ) , both phosphoric acid and edta ( groups 5 and 6 , respectively ) decalcified the underlying dentin , facilitating the diffusivity of the nano - prime through the decalcified dentin surface . phosphoric acid widely opened the dentinal tubules , resulting in the formation of numerous long and funnel - shaped , resin tag extensions ( figure 3 ) . furthermore , a thick hybrid layer with filler distributions at the bottom of and within the hybrid layer as well as , around the orifices of the dentinal tubules was evident ( figure 3 ) . on the other hand , edta did not aggressively decalcify the dentin surface or widely open the dentinal tubules , resulting in the formation of long and thin resin tags , thinner hybrid layer , and much less filler distributions ( figure 4 ) . it was reported that the formation of the hybrid layer and resin tag extensions is crucial to resist the acute de - bonding stresses23 that could occur during bond strength testing.24 as different hybrid layer thicknesses were evident in sem , given three different dentin conditioners used in this study , our sem results suggest that the length of the resin tags played a major role in the enhancement of the sbs of nano - filled rmgi for groups 5 and 6 , when compared to all tested groups . in the experimental design of this study , one of these two groups , the nano - filled rmgi was bonded directly to the dentin surface with neither nano - primer nor dentin conditioners ( group 2 ) and the other was the nano - filled rmgi bonded directly on conditioned dentin surface with 25% polyacrylic acid for 10 seconds without the use of the nano - primer ( group 3 ) . these two groups were included to assess whether nano - filled rmgi has the self - adhesiveness property , which is one of the main advantages of the conventional rmgi . unfortunately , based on the sbs results of this study , nano - filled rmgi lacked the self - adhesiveness property . this result , specifically for group 2 in this study , was consistent with the results of coutinho et al,3 in spite of these studies differences in bond strength testing methodology . they reported that when nano - filled rmgi was directly bonded to dentin , its bond was limited to the roughness that resulted from dentin surface preparation . therefore , groups 2 and 3 were omitted from the sem section of the study . advances in material - manufacturing technology might improve some properties , but on the other hand , it could ruin other important and unique properties of the materials . based on this fact , additional efforts should be made to carefully assess these newly introduced materials . based on the results of the current study , several conclusions can be drawn : 
 the pre - conditioning step effectively improves bonding of the nano - filled rmgi to dentin.the self - adhesiveness of nano - filled rmgi was not proved in this study , even with the use of 25% polyacrylic acid conditioner.the enhancement in bonding of the nano - filled rmgi depended on the presence of long resin tag extensions in the underlying dentin . the pre - conditioning step effectively improves bonding of the nano - filled rmgi to dentin . the self - adhesiveness of nano - filled rmgi was not proved in this study , even with the use of 25% polyacrylic acid conditioner . the enhancement in bonding of the nano - filled rmgi depended on the presence of long resin tag extensions in the underlying dentin . nevertheless , further studies should be conducted to assess the durability of nano - filled rmgi bond when a pre - conditioning step is used .
objectives : the purpose of this study was to evaluate the effect of the pre - conditioning step using different dentin conditioners on the shear bond strength ( sbs ) of the nano - filled resin - modified glass ionomer ( rmgi ) to dentin.methods:twenty-four human molars were used in this study . 
 subsequent to teeth sectioning in the mesio - distal direction and creation of the smear layer , the teeth were divided into six groups with and without the pre - conditioning step . 
 dentin surfaces were restored with rmgi cylinders and the specimens were stored in distilled water at 37c for 24 hours . 
 sbs test were performed using a universal testing machine at a crosshead speed of 0.5 mm / minute . 
 statistical analysis was performed using one - way anova followed by duncan s multiple range test . 
 rmgi / dentin interface was evaluated using sem operated at 12 kv at 2000x magnification.results:35% phosphoric acid and edta yielded significantly higher sbs ( 12.01.8 and 11.94.4 mpa , respectively ) , compared to all tested groups . 
 in addition , 25% polyacrylic acid ( 8.63.1 mpa ) reported significantly higher sbs than the nano - filled rmgi when applied according to manufacturer instructions ( 5.52.2 mpa ) . when nano - filled rmgi was applied without its nano - primer , or directly over 25% 
 polyacrylic acid conditioned dentin exhibited 100% pre - test failure , and their sbs were expressed as 0.00.0 mpa . 
 sem revealed that the nano - primer was unable to decalcify and hybridize the dentin surface.conclusions:based on the results of the current study , the pre - conditioning step effectively improves the sbs of nano - filled rmgi to dentin . 
 the self - adhesiveness of nano - filled rmgi was not proved in this study .