developing new techniques to increase bond strength in resin restoration is necessary to reduce marginal microleakage and discoloration due to failure in marginal integrity .
so , measuring the enamel and dentin bond strength is important in the evaluation of mechanical fracture and therefore the prognosis of dental treatment .
the application of alternative methods such as laser irradiation has been increased in restorative dentistry . among different lasers ,
the er : yag laser was approved by fda in 1997 for caries removal , cavity preparation and conditioning of the enamel or dentin .
this laser with a wavelength of 2940 nm and its high coefficient of absorption in water and hydroxyapatite is more effective than the other lasers for preparing hard tissue .
it produces minimal thermal effect on the tooth structures and surrounding tissues in comparison with other dental lasers especially when water spray is applied .
this laser produces an irregular pattern on the surface of the enamel that can improve retention for bonding of restorative material .
advantages such as biocompatibility , adhesion to tooth structure , fluoride release , reduced microleakage and lower polymerization shrinkage have led to wide use of glass - ionomer materials in restorative dentistry .
different parameters and methodologies have guided to controversial results about the effect of er : yag laser on the shear bond strength of dental materials to enamel .
souza - gabriel et al . in assessing the shear bond strength of resin - modified glass - ionomer to er : yag laser treated surfaces concluded that conventional bur - prepared samples provided better adhesion than samples prepared by er : yag laser . on the other hand ,
visuri et al . found higher values of bond strength in the irradiated surface by er : yag laser compared to those that were bur - treated .the aim of this study was to evaluate the shear bond strength of resin modified glass ionomer to enamel in er : yag laser - treated surfaces in comparison with bur - treated surfaces .
all teeth were stored in distilled water with 0.4% thymol for 1 month to reduce the formation of microbial plaque .
the teeth were embedded in clear acryl ( repair material , dentsply international inc . ,
milford , de ) 2 mm under the enamel - cementum junction ( cej ) .
the buccal surfaces were flattened on the enamel level by carbon disc with 3 mm thickness and12.5 mm diameter .
the first group was irradiated by er : yag laser ( us2940d , deka , italy ) .
this laser operates at a wavelength of 2940 nm accompanied by water and air spray .
the laser irradiation was done with an average output power of 3.5 w , energy of 350 mj and frequency of 10 hz with a pulse duration of 230 s in a sweeping motion about 4 mm above the surface .
the distance from the enamel surface was controlled by fixing an endodontic file to the laser handpiece .
the spot size of laser was 1 mm . the second group was prepared by carbide bur ( 6 flutes and spiral angle of 30 ) under water spray in sweeping motion for 5 seconds .
the dimension of the area , which was conditioned by two methods , was approximately 2 mm  2 mm .
after surface treatment of the two groups , the conditioner ( 40% polyacrylic acid ) was applied for 30s .
then , self cure resin modified glass ionomer ( fuji ix gc corporation , tokyo , japan ) was placed on the surface in a 2.8 mm diameter and 3 mm height cylindrical mold .
all the samples were placed in 37c distilled water for 24 h to provide the final setting of the restorative material .
the samples were subjected to universal testing machine ( zwick , germany ) in order to measure the shear bond strength at a speed of 0.5mm / min and a 50 kgf load until fracture .
the results of the two groups were analyzed by t - test at 0.05 confidence level .
the mean and standard deviation of carbide group and er : yag laser group were 4.411.62 mpa and 6.751.99 mpa , respectively .
there was significant difference between the shear bond strength of the lased and the bur group ( p - value=0.01 ) .
according to the microretentive pattern obtained by er : yag laser irradiation , which is suitable for adhesion , in this study the shear bond strength of an rmgi to lased enamel surfaces in comparison with bur - treated surfaces was assessed .
bond strength testing as a laboratory methodology has been proposed to evaluate the adhesion capacity of dental materials.the shear bond strength test is a simple procedure for experimental evaluation and also a screening mechanism for predicting clinical performance [ 9 , 10 ] .
laser technology has been presented as an alternative option to replace the conventional high speed turbine .
this technology offers the patient comfort by reducing the pressure , heat , vibration and noise produced by a rotary instrument . using laser for etching enamel
application of phosphoric acid for etching the enamel makes the surface more susceptible to caries because of demineralization of the superficial layer .
the physiochemical changes by laser etching reduced the acid attack and the risk of caries .
it may be related to changes in ca / p ratio , reduced carbonate and pyrophosphate formation accompanied by the reduction of water and organic component [ 1315 ] .
er : yag laser acts on the dental substrate by thermo - mechanical ablation and vaporization of the water content which causes expansion followed by microexplosion that produces the ejection of both organic and inorganic tissue particles [ 1618 ] .
therefore , it blocks the intra- and interprismatic spaces and restricts material interdiffusion into the enamel surface [ 19 , 20].the micromorphology of laser treated surface shows less regular and homogeneous aspects with some fissures occurred in subsurface resulting from heat generated during irradiation .
resin modified glass ionomer has many advantages and has been employed in dental clinic because of the physiochemical adhesion to the enamel and dentin . for a long period , it can release fluoride ions in adjacent enamel and may absorb fluoride from other sources like toothpastes .
it also presents good adhesion , marginal seal and reasonable esthetics [ 22 , 23 ] .
most of the studies that evaluated the shear bond strength of laser treated surfaces in comparison with bur - prepared enamel showed higher bond strength in the bur - prepared group .
measured the shear bond strength of resin composite to enamel treated by different energy intensities and frequencies of er : yag laser compared to phosphoric acid and concluded that acid conditioning of the enamel showed higher bond strength than laser .
investigated the shear bond strength between light - curing nano - ionomer restorative and enamel or dentin after acid etching , after er : yag laser etching or after combined treatment .
they concluded that etching with acid phosphoric increased the shear bond strength , but the laser group showed a lower bond strength   that was in contrast with the results of the present study .
when er : yag laser was used for surface treatment , compared to bur preparation it produced no smear layer leading to increased surface wettability and producing tag formation .
on the other hand , turkmen et al . showed that er , cr : ysgg laser etches the enamel surface more effectively than phosphoric acid that is in agreement with our results .
the higher values that were achieved from the laser group may be contributed to micro irregularities produced on the surface .
there is need for more studies which evaluate the interaction of laser prepared surfaces with new generation of glass ionomers like light - curing nano - ionomer restoratives .
based on the results of the present sudy , er : yag laser can be an alternative device for enamel preparation in restorative dentistry .
 objective : the purpose of this study was to evaluate the effect of er : yag laser on the shear bond strength of resin modified glass ionomer ( rmgi ) to enamel.materials and methods : twenty extracted caries - free human premolars were selected . 
 the teeth were embedded in acrylic resin . 
 the buccal surfaces of each sample were ground to plane enamel with carbonated disc . 
 the teeth were randomly divided in two groups . in the first group 
 , the surfaces were treated by er : yag laser ( 350mj/10hz ) . 
 the second group was prepared by carbide bur . 
 fuji ix rmgi was adhered to surfaces of the samples in both groups in rod shape . 
 the shear bond strength of samples was measured by a universal testing machine . 
 the results of the two groups were analyzed by t- test.results:the means and standard deviations of shear bond strength of the laser - treated group and the bur - treated group were 6.75  1.99 and 4.41  1.62 mpa , respectively . 
 there is significant difference in the shear bond strength of rmgi between the two groups ( p - value=0.01).conclusion : the laser group showed better results . 
 er : yag laser can be an alternative technology in restorative dentistry . 
INTRODUCTION
MATERIALS AND METHODS
RESULT
DISCUSSION
CONCLUSION